i only play d2, so keep that in mind if comparing to posts above. I +1 in 4 situations (NEVER +2):
1. if i know (read: can make an educated guess) that a team is going to cheat back (-3 or more) against me and i want to keep them honest, i'll +1 one of my guards.
2. in a similar vein, if i want to avoid or take advantage of a potential shift in half time settings (i.e., my bigs are loaded with distro, and i know that either opp adjusts every half time or if i'm winning, etc), i may be able to exploit a 2nd half opening), so i'll +1 my best 3 pt shooter.
3. when i have a player that's unathletic (< 40) but fast (70+) with good per (70+) matched up against a more athletic but slower defender, i'll +1.
4. Against the 2-3 zone i like to put in a per shooting sf and +1 one or both of my guards.