I can only speak for my LL but in general, look at both the content of your FS and the titles and descriptions you use.
Title: For example, If you have a site with a brunette teen with perky tits, you could submit in either 'brunettes', 'teen' or 'perky tits' - of course, I like to see a description that fits the content and the cat.
Same content - different cats.
Cat: Perky Tits
Title: Perky Little Tits
Description: Skinny girl with perky little tits massages her hard nipples.
Cat: Brunettes
Title: Cute Brunette Model
Description: Sexy brunette model plays with her hard nipples for the camera.
Cat: Teen
Title: Firm Teen Body
Description: Hot teen model massages her firm tits and hard nipples.
Remember that as an LL owner, I'm looking for words that not only accurately describe the content, but also words, properly used, that help each cat in SEO.
And do a check on search engines on how often terms are searched on. According to
http://www.submitexpress.com/keytracker.php "alluring teen" gets searched on a whopping 0 times - but I see it on titles on FS's and galleries all the time. Whereas, "sexy teen" gets around 2,500 daily searches per tracked service. Would you search for alluring teens?
I'll often put a site in the cat the not only fits best for the content, but one where the description is accurate for the cat. If you're lucky, I'll put it in all the cats that fit.