TJ-
I know you asked about styrene, but if you're interested in really getting into scratchbuilding, you might want to consider wood. You can purchase sheets of scribed siding that simulates common construction methods (like clapboard, board and batten, etc.). Micromark (
http://www.ares-server.com/Ares/Ares.asp?MerchantID=RET01229&Action=Catalog&Type=Department&ID=73) has pretty good prices -- for picking sizing a good rule of thumb for HO is that 1/8" is close to 1 scale foot. Local hobby stores also will carry scribed siding (in both wood and styrene).
The advantages of wood are that 1) it looks like wood (so you get wood grain without having to do anything special ) and 2) it's easily glued with common yellow carpenters 'glue or superglue. Styrene needs solvent based cement (which your LHS can help you with).
When I got started seriously in scratchbuilding, someone alerted me to the old Campbell's kits. They are wood and just a step down from full scale scratchbuilding -- basically you get detailed instructions and all the wood, windows, shingles, etc. that you need. These really are a great way to cut your teeth with basic scratchbuilding techniques if you haven't had much experience with it, and all the needed parts are right there in the box so you don't need to mess around with guessing how much wood you need. Campbell's is sadly not making kits currently (although rumor says they will be back soon), but kits are available on ebay and hobby shops still occasionally have them on the shelves. Walthers.com also shows some still available. They are pricey, but a lot of the really great current scratchbuilders got their start with them.
Of course, there are also some really good tutorials in the Academy section of the gauge, which can walk you through scratchbuilding from start to finish...