I know there are a ton of websites and blogs out there on this topic - I know this because I've pretty much read them all. Wow! Who'd have thought there could be so much information, and so much of it contradictory, on one subject?!
But here's a couple little things that I've personally found so far. Now, granted I'm not "published"... yet.... but, none-the-less it's my 2 cents worth at this point for those interested.
1. Research Agencies and Agents, but believe it or not, Goggle is not the best way to find them. Personally I use a free account with Author Advance: http://www.authoradvance.com. It has an awesome search function and has close to 2000 agents listed within it from all over the world. Whether or not it's an exhaustive list - who knows, probably not, but I do know that it'll take me some time to filter through all of them and hopefully I'll have representation before I get even half way through. (Got me fingers crossed!)
2. Once you have their website - use it! Go to each agents' profile to find out what they are looking for and also find out what their submission guidelines are and again - use it! You don't need to be super crazy strict about "following the rules" - but at the same time, not much point on sending a Query on a Fantasy novel to an Agent who only handles non-fiction memiors.... and even worse if you get their name or gender wrong! So do yourself a favour and do your research.
3. Which brings me to the point of getting things wrong..... I couldn't believe it when I started finding out just how many Query Letters get tossed out by an agents assistant or intern because of typos! So again, do yourself a favour.... read, re-read and re-read again - then get your friends and family to read it too - to make sure you've got it right.