I think a big preempting mistake beginners make is preempting 4H or 4S with an opening bid in 1st or 2nd position. I completely agree with destiny in #8.
Modern players seem to preempt a little lighter lately, although I can't disagree with Irdoz.
I don't worry about strength of suit as much as Irdoz, because I feel preempts are so effective. The occassional disaster is worth many good results.
Here are the rules I tend to use:
MP:
Weak 2:
1st position: Any 6 carder, 4-10 HCP. Don't care about voids or 4 card majors.
2nd position: Any 6 carder, 4-10 HCP. No 4 card major.
3rd position: Any 6 carder or good 5 carder, 4-10 HCP. Don't care.
4th position: Good 6-carder, close to opening bid.
3-level:
1st position: Any 7-carder, less than 10HCP. Don't care about 4-card major.
2nd position: Any 7-carder, less than 10HCP. No 4-card major.
3rd position: Any 6 or 7 carder, less than 10 hcp.
4-level: 8-carder with same rules as 3-level.
IMP:
Weak 2:
NV: Same as MP
Vul: Good 6-carder only (2 of top 3 honors)
3-level:
NV: Same as MP
Vul: 7-carder with 2 of top 4 honors.
4-level:
NV: Same as MP
Vul: 8-carder with 2 of top 4