I usually vote up a question if it meets any of these conditions:

- I can answer it with the information given and I don't see any reason why it should be closed
- Someone else has answered it well or I know someone else will be able to answer it and I don't see any reason why it should be closed
- It seems to be a useful/interesting question and includes enough information to answer it in theory and is on topic (I upvote even if voting to close as dupe)

I usually vote down a question if:

- it is a rant
- I think it is unanswerable with the information given after allowing time for OP to respond to comments 
- I don't think it could help anyone else

I find questions to vote on:

- by watching the feed, reading the front page
- doing reviews
- following links posted in comments, in answers, and in the sidebar to linked/related questions
- using the stats page in mod tools
- doing cleanup (I use [a data query](http://meta.askubuntu.com/questions/16236/cleanup-project-using-sede-query), my old comments, my review history)

Rewards for voting:

- what goes around comes around ;) or rather, if you like getting votes, then maybe you should be giving votes?