here are my lessons:
1. just blu-ray.com for PQ / AQ on releases before buying
2. don't buy a film just because it's the hot item being talked about in the forums
3. (this kind of bypasses #2) if it's special packaging that may not be around very long (ala: Fight Club slipcover, thank god i bought it day 1 since i rarely see the awesome slip cover anymore) BUT IT ASAP!
4. as others have mentioned, when you buy something off amazon or any other site, don't expect to always get slip covers / digital copies / special discs outside of cases (The Office, Dexter)
5. prices will drop on all releases. only about 5 films are worth spending $30 or more on
6. when buying 1 item on "new release tuesday", don't buy a bunch of other new releases "just because i can"
and i'm sure there are a bunch more. but these are my main ones.