Semi-rares rock!!
http://kol.coldfront.net/thekolwiki/index.php/Semi-Rare_AdventuresThe borg take advantage of every SR (semi-rare) they can get their assimilating little hands on! We recommend you do too. Dalgar has a nice thread full of spading and info on this located here
http://forums.hardcoreoxygenation.com/viewtopic.php?t=4077 .
To make it easier on yourself, use your 0 skill (the one you're given when you create a character, or when you ascend into a new one) to help you. If you're hco, your very first sr will occur roughly between turns 100 and 117. Once you find one, cast 20 of your 0 skill (100 advs worth). When it runs out, it's time to go hunt your sr! If you're in hcnp, your first sr will occur between turns 165 and 196. If I'm in hcnp, I'll cast my 0 skill 32 times (160 is close enough), then eat a fortune cookie when it runs out. One should be sufficient to determine when your next sr will be. If you're unlucky and get a slew of small numbers, eat one more

When you pick one up, cast 32 of your 0 skill, rinse repeat.
If you don't have the mp, or are doing a 100% familiar run that won't give you tons of free mp, then try cans of hairspray (33 for hco, 53 for hcnp) or ben-gal balm, whichever will be most beneficial for your class that run.
Bad Moon semi-rares follow the pattern for hcnp fairly closely. There isn't much info for softcore yet, and the range for casual seems to be a bit larger, 155-199.
You might want to keep track of which sr you pick up when as you can't get two of the same kind in a row, here's a borg sample...
Turns Played (this run): 174 (lunchbox)
Turns Played (this run): 349 (KGE)
Turns Played (this run): 527 (lunchbox)
Turns Played (this run): 707 (mystic shell)
Turns Played (this run): 883 (pet snacks)
Good luck, and happy semi-rare hunting!