1. Make the resource box another paragraph for the article - Most know this tip, but I always try to include everything. Basically, write a 400 word, 5-6 paragraph article. Then copy and paste all but the last paragraph into the article box, and copy and paste the last paragraph into the resource box. What's great about this method is that it allows you to embed links within the article, which people are much less suspicious of. Give it a try, see how it works.
2. Top author - Many article directories allow you to view who the top author are for each category and subcategory. So, climb as high as you can in the author rankings for your category, then mention it in your resource box. This works especially well on EzineArticles. If you become the top author, change your profile and mention it EVERY resource box. It instantly credits you as an expert, and your clickthrough rate will increase dramatically. A bonus to this is that your article views will increase, as people frequently see who the top authors are and read their articles.
3. Solve their problem - If you're any good at marketing then your product will solve a problem that your reader has. While you should focus on the benefits to the reader of the product, specifically say how it will solve their problem. Focus this on the keyword, and you'll be golden. For example, if your keyword is 'get rid of acne on your nose', write your resource box about how the product will get rid of the acne on their nose. Make sense?
4. Not a bio - this isn't really a tip so much as a warning. Most readers simply do not care about you or your pseudonym. DO NOT use your resource box as a bio box. The only thing that you should mention about yourself is if you have a degree relevant to the niche, or you're the top author.
5. Links - make one link an anchor text link for your keyword, and make one link a call to action link. So, for the earlier example, one link would be 'get rid of acne' (because 'get rid of acne on your nose' is too long a link for many article directories; always use the broad keyword if you have to shorten), and one link would be 'get rid of the acne on your nose right now, go to www.yourwebsite.com immediately!' A bonus to this method is that if someone just copy/pastes your article instead of the proper publishing method (which is very common), your website link will still work. If you had two anchor text links, they would both be dead.
Hopefully these were helpful, let me know of any that I missed. I'm always open to learn new tricks.