OK I'll let the gurus handle most of that question, since I'm new to SWG myself, BUT I know you need to add CYA. At Wal-Mart it is called Stabilizer (and some other places), but it is 100% Cyanuric Acid in the ingredients. You need to read the Stickies page on this forum, and maybe alot of the posts in the SWG section, that helped me a whole, whole lot.
After discussing CYA with the gurus here, I put it straight into my skimmer (but you have to remember NOT to backwash for a week after adding it this way, so if you're having to backwash more often than once/week right now, put the CYA in a sock and let it dangle in front of a return, realizing that it takes MUCH longer to dissolve this way. Otherwise, put it - slowly - directly into the skimmer and don't backwash for a week, then test the CYA again after that week).
-Added this- Check JasonLion's calculator to determine how much CYA to add.
The CYA will help stabilize your chlorine usage. Most people here seem to recommend 60-80 CYA level, and you should check your SWG manual.