Sites like reddit and sites on the StackExchange network have posts that can be voted up or voted down to rate their usefulness.
On some posts that have many votes in a certain direction, many users may vote one way based on the vote count instead of the content on the post. How can this pile-on voting be combated in the community?
You first need to verify the actual problem at hand. See, voting behaviour is something you should affect as little as possible. Users should always be free to vote however they like, as long as they are not exploiting flaws in the system.
The phenomenon "pile-on voting" can have any number of reasons: It could all be genuine votes, regardless of the score. It could also be a snowballing effect: A post that already has upvotes will gain more exposure and therefore more upvotes. For downvotes, the very same attention applies on a psychological level (rather than a system boost): "Why is that downvoted so heavily? Gotta check it out!". Or, in the case of Stack Exchange, the post might be pushed to review queues or be brought to users' attention through some other way - which increases exposure and therefore more downvotes.
As for solutions, you have very little in terms of community options. You can try educating people - PSAs, guidelines, something to tell users how they are supposed to use their votes (which, again, runs a bit contrary to the idea of being free to vote how they please.) You need to point the community as a whole at these resources.
At the end of the day, this kind of voting behaviour tends not to be actively harmful to the community. It is not at all easy (or, even possible) to define, detect, and identify this kind of vote(r) - which is necessary before you can consider addressing individual users about their "wrong" votes. As such, I cannot honestly recommend taking much action about this beyond attempting to educate the community.
If, however, you have influence on the technical side of things (and you truly deem this voting behaviour to be an actual problem), you might want to rethink the score feature entirely. If you want to show a raw score, that also means people get to see the raw score, and subsequently use it in their voting behaviour.
You can make people value their own votes more. Someone who sees a post that already has lots of votes in a given direction might not want to vote in that direction if their votes have a certain value to them. There are a few ways of doing that: A low amount of votes per time unit, or making the vote have a big impact on some feature on the site.
As I said above, you might also consider not showing the raw score. An option would be fuzzing the score somehow (a "topic heat" meter comes to mind). The implications are, however, that your voting system becomes largely useless because you just removed a core element.
One option which isn't widely used (it has some significant drawbacks) would be to use the vote count for ranking internally, but don't actually display the vote count to the user until after they vote.
The advantage is that it forces users to consider the post on its own merits while still benefiting from the ranking associated with community experience. The big downsides though are that it greatly complicates determining how much better a post is seen by the community. If the user is in a relative hurry, they have no way to know if the post is barely considered any better or is considered substantially better until after they make a choice about it.
This may also actually discourage voting in some cases as well. You need to carefully consider why pile on voting is occurring and consider if you actually believe it is a problem. Often, a user may like the way a post is written and find it helpful, but not know enough to judge the correctness, and therefore takes no action. If they see the post has relatively few downvotes and a lot of upvotes, they are more likely to assume the post is correct based on community approval and contribute their vote. This may be an advantage or a disadvantage depending on what you are going for, but it is something that needs to be carefully considered.
