Semibluffing is such a situational play that it is very hard to just give tips. I tend to semi bluff with 1, or at most 2 villians in the pot with me.
There is an EXCELLENT chapter on semi bluffs in Skalnksys : The Theory of Poker.
You should grab that book and check it out.
One tip though is I look at my villians effective odds. If they are short stacked, I don't even bother with a semi bluff, but if my villian has a huge stack and it can get me doubled up if I do hit my straight of flush, I more than often semi bluff.