Crocodiles have supposedly remained unchanged for millions of years, and several other species are considered as "living fossils". How do such species remain so constant over time given that they will have had so much time to accumulate new mutations?
Evolution is a process of change by four mechanisms; mutation, migration, drift, and selection.
You are correct in thinking that, because crocodiles have been around for a long time, they could have accumulated many new mutations in that time, relative to other more recent species. However, mutation is only one of the important mechanisms underlying evolution.
How different are ancestral and modern crocodiles?
How can they remain so unchanged?
Genetic variation may have been low in the ancestral population, this would reduce the potential for evolutionary change, as most change would have to occur through new mutations. It seems the populations of ancestral crocodiles were quite small, such that a genetic bottleneck may have occurred (which would reduce genetic variation). Mutation rates seem to be relatively low in crocodiles (also see here) which would reduce the rate at which novel mutation occurs, reducing the potential for evolution.
Note that many mutations will be neutral in their effect (or "synonymous") so won't have an obvious phenotypic effect, so there may be substantial evolution at the genetic level despite the phenotypic similarity.
Low rates of evolutionary change could suggests some other things may have also played a factor. Given that the populations have been through multiple genetic bottlenecks, genetic drift could have slowed down rates of evolution eroding genetic variance, removing rare mutations from the population.
If selection has been fairly constant over time then there is less chance that changes will occur. If selection were to change and favour new adaptations then these are likely to spread, but if selection remains fairly constant over time then it will continue to favour the same mutations. After a long time of consistent selection it is likely that most mutations will be deleterious (have a negative effect) and be removed from the population by selection. Darwin suggested that living fossils could occur because the environment they are in has remained fairly constant (from this link).
Crocodylomorpha were actually once a lot more varied than they are today, so their group isn't immune to change or evolution.
The tongue in cheek answer is to say, there were a lot more forms of crocodile in the past so chances are the single form we see today would look like one of them!
A better answer is: Species often evolve to fit a niche, they become specialised in their form both externally and internally. As long as this niche stays the same and neighbouring niches remain filled, the species will only become more suited to its niche. Over time you'd expect new mutations which made an individual fitter to become rarer, or have a smaller impact on survival so be selected for less strongly, thus the longer a niche exists, the more stable the form of animals living in it will become.
I think in the case of crocodiles in particular, this is a question of body type. Crocodiles have fixed jaws, meaning that they have lost the mechanism used to move their lower jaw from side to side. This simplification means that they can exert a huge amount of power, making it a great adaptation for their particular strategy of hiding in water and ambushing large land mammals that drink from it.
However, it also means that it's very hard for them to adapt to any other niche. Anything that would require eating smaller prey, or eating on land, would be untenable, because crocodiles can't chew. (Instead they close their mouth and rotate their whole body in the water to rip chunks of flesh off.) They can't easily re-evolve the ability to chew, because the mechanism needed to do that is pretty complicated, so their fixed jaw is more or less "locked in" evolutionarily.
So crocodiles have remained constant because they are very well adapted to a particular niche; because they've lost features that would allow them to adapt to other niches; and because the niche they occupy has been around for a very long time.
Personally I think these are the primary reasons. The low mutation rate mentioned in the other answers seems to me more likely to be effect than cause. If you are very tightly adapted to a particular niche then there is less advantage in genetically exploring other possibilities, so the descendants of individuals with lower mutation rates would have an advantage over those with higher mutation rates. This would provide an evolutionary pressure for a low mutation rate, which might explain the observation. (But this last paragraph is speculation on my part. I work on modelling evolution and I know that this kind of selection on mutation rates can occur, but I have no evidence at all about whether it happens in crocodiles.)
I think rg255 and Troyseph pretty much nailed it, but another thing to consider is the crocodilian's habitat. All surviving forms are aquatic, with at least one species - the marine crocodile - at home in the sea. In addition, most, if not all, crocodilians live in tropical or subtropical regions.
In fact, many "living fossils" are associated with tropical forests or the sea. The marine coelacanth is one of the most celebrated living fossils, for example. The most primitive living fishes also include the lungfishes, which are generally freshwater tropical species.
The tropical monotremes are considered the most primitive living mammals (and the platypus is semi-aquatic to boot).
Their niche and "biology" also make it relatively difficult for crocodiles to exploit other niches. Even if they were't tied to water, one could hardly imagine a crocodile climbing over a high, cold mountain pass to reach a lush, tropical forest on the other side.
It's worth noting that all the terrestrial crocodilians have died out, leaving only their aquatic or semi-aquatic relatives.
Here's one interesting source -- 12 of the most astounding 'living fossils' known to science.
Most of the species listed (including the coelacanth) are marine organisms. It also lists crocodiles, and I was surprised to learn that crocs evolved from marine organisms themselves. (See This handsome sea creature is where crocodiles came from).
Similar lists include tropical forest creatures, like the African okapi and Australasian cassowary. Note that rhinoceroses and tapirs now occur only in the tropics (and perhaps subtropical regions for African rhinos), now that the more recently evolved woolly rhinoceros is extinct.
Well, two things.
First the assumption that creatures that are living fossils are strong as a species isn't strictly true. Evolution; especially in relatively long-lived animals like crocodiles and sharks (compared to dogs, houseflies, j-walkers, etc.) is such a long process. It isn't factually verifiable that they are doing well as a species, only that they are in a stable food chain.
Second; and User23715 raises all these point, animals that aren't exposed to evolutionary motivators (mutagens, predation, and habitant change) won't have as much genetic divergence. A crocodile won't benefit from a Darwinian advantage if it doesn't provide enough of a survival advantage to eventually fuel a divergent evolution. Any dominant trait needs to be aggressive enough to stay dominant. In a simple dominant-recessive model it would need to be twice as potent for hundreds of generations (close to 2000 years for crocodilian life spans and birth rates).