Smartening up your grammar skills doesn't have to be a boring affair - in a previous post I linked to humor site The Oatmeal for their guide to correct apostrophe use, presenting the rules of one of the most oft-misused marks of punctuation in a series of bizarre and surreal cartoons.
Lifehacker.com - a site mostly known for its tech articles - has recently collated and published a list of the best sites to go to for brushing up your grammar skills without having to force yourself to read through dull, instructional texts and rules. The collection of sites all aim to educate while entertaining, and include favorites such as The Oatmeal, Grammar Girl, and blogs where embarassing errors are uploaded and showcased for the obsessive grammarians out there to laugh at - sites like Apostrophe Abuse and The Society for the Promotion of Good Grammar.
All of these resources will help you if you ever have problems like remembering when to use an apostrophe and when not to, which variant of their/there/they're to use, whether you mean e.g. or i.e., and other common confusions.
Alternatively, you can have a look over my previous "annoyances" posts, which aim to help you avoid common mistakes that tend to irritate anal obsessive compulsive people... like me.