govtec, didn't see your other post before making mine – sorry. I'd pulled up the thread last night, then posted this morning, so missed your post since I didn't refresh. I was taking quite a few CLEPs (want to say I took around 10 total?), so I got pretty efficient with it. Generally I'd take one every 2 weeks or 2 every 3 weeks. The best study materials in my opinion are the REA guides – example for history: https://www.amazon.com/CLEP-History-United-States-REA/dp/087891272X/ref=sr_1_3?ie=UTF8&qid=1417736233&sr=8-3&keywords=rea+history+of+the+united+states These books pretty much cover it all, or at least let you see where your weak areas are.
However, unless you're a history buff, history isn't necessarily one of the easy ones since there's so much information, so many dates, etc. If you're just looking for quick credits, Intro to Sociology and Intro to Psych are both good if you haven't taken a similar course (any Psych 100-level course would probably count as a duplication for Intro to Psych…). This site has pretty good info in general, but here's their difficulty rating for the exams: https://www.free-clep-prep.com/clep-difficulty-list.html
One I'd strongly suggest you consider is Analyzing & Interpreting Lit if you don't have a similar course and just need any credits. I really really don't like Lit, but I would still recommend this one because the REA guide is all you need to prepare and it's 6 credits. I think I studied for this one around a week before deciding that anything I didn't know, I wouldn't be able to learn, so I might as well fail and move on, but then surprised myself with passing (it ended up being my best CLEP score – 71 I think it was).
By the way, if you're looking to actually learn something useful at the same time, you could consider DSST tests too: https://getcollegecredit.com/ They're primarily used by military but are also primarily business-related tests, so would potentially have more useful subjects.