A good strategy is get Gheed's charm - the discount on vendor prices works on gambling too! That means a lot since gambling is expensive. Also, get an Edge runeword (you're going to have to use the runeword mod since it's bnet only, normally), which is another -15%. You'll be gambling with a 30% discount with these two items!
I agree with you on what to gamble. Go for jewelry and the circlets line. Personally I just forget the rings, because I tend to find that they're common enough if you spend some time mfing on hell meph, and it just takes too long to gamble the best ones (soj/bk).
I like gambling on amulets for the +3 skill ones, they're very useful in prebuffing certain trees. It's also a good way to keep looking for that +skills and teleport charge amulet which is really great for non-sorcs, or a +skills and magic find one.
Circlets can be truly amazing at higher levels. The ones with +2 skills, resist all, faster run/walk, +20 dex/strength, dual leech....very nice. Even real early on circlets are great, I remember in act 2 with nontwinked amazon at level 19 I gambled one with +1-111 lightning damage, 23% magic find, +10-20% resist lightning/poison, I think I still have it
Everything else is just too expensive to gamble or you can find/buy elsewhere easily. Jewelry is nice to gamble because the price is always the same, and circlets just have surprisingly great mods.