The two biggest things I learned from this site:
1) Have the right tools and supplies. The two stores I tend to frequent when looking for this type of stuff are JoAnn Fabrics and Michaels (where October is Jewelry Month! Check out the events they have at your local store). They both have pretty large jewelry making sections now and both offer similar pricing and coupons! Who doesn't love a good coupon?
2) It's all about proper balance of the posts. Sometimes the center is great for a clip, but not for the weight of a post. Before you attach a post be sure to look in the mirror and see where the earring would rest depending on where you place the post and use that to judge your placement.
Has anyone else tried to convert clips themselves? Any tips to share?