The first thing you need to is be willing to embrace the mainstream. Hip hop is structured entirely differently to rock, and if you want to listen to something other than rap for white college kids (Aesop Rock, Jurassic 5, Sage Francis, Atmosphere etc.), you’ll need to be prepared to give a fair listen to anything, no matter how big it sells. For instance, the only hip hop record to make the list of the top 10 best selling albums for the first 6 months of 2006 was T.I.'s King, and it’s also one of the year’s best hip hop albums.
For a start, the lines between mainstream and underground in hip hop are less easily defined - Jay-Z shouts out underground icons like Talib Kweli and signs the Roots to Def Jam. Secondly, regionalism is very important in hip hop, and an artist who is a local hero selling CDs out the trunk of their car one day can be an international superstar the next if their region attracts the right amount of attention.
There’s also the problem that if you are 2-3 years behind, what you’re listening to right now is basically the peak of an underground that really fell off straight after. In '06, the only real interesting “conscious” albums have come from The Coup and (apparently, though I haven’t heard it yet) Mr. Lif, which is very different to how things were a few years ago. (Also, The Roots new record, not out yet, is being talked about as their best in years, but they’re on Def Jam now, so it hardly counts as being underground). 2005 was even worse for underground rap - almost entirely regressive or didactic or both. There’s no dominating underground force right now the way Rawkus and Def Jux used to run the underground game. Not that the best artists are doing huge numbers, but they have far more in common with the biggest artists than the backpackers and all that. Probably the most anticipated hip hop release of the year right now is the Clipse’s new record, and they’ve got production from the Neptunes and rap about dealing.
The best way to find out about what’s hot is cheking out blogs - try Nahright.com, Sohh.com and the XXL blogs, and check out the links from those sites. It’s also a good idea to get up on local scenes. The Bay Area, Miami, Houston, Memphis, Atlanta and Baltimore all have thriving scenes at the moment, and a bit of research - even sniffing around on Wikipedia a bit - will turn up some good names. (New York isn’t so hot at the moment, but the Dipset still has their hardcore fanclub, and there are people who think Papoose is the second coming, even though I don’t see it. Then again, he just signed a $1.5 million contract, so maybe he will do something worthwhile).
If you’re in a major city, you can hunt out the guy’s selling mixtapes, which are always a good entry point for an artist. They’re always cheap and throwaway ($5) and have a lot of junk, but often some good stuff as well. Lil’ Wayne’s Dedication #2 mixtape is probably one of the best hip hop albums released this year.
And if all else fails, just see what Wu Tang Clan members have albums out, and go from there. 
