Any microstock photographer will tell you that good keywording is absolutely vital to being successful in the industry, so naturally I'm going to tell you the same thing. Thankfully, the fact that it's important doesn't mean it's difficult to do; though, you may find it's one of the more tedious jobs in your workflow.
Firstly, you need the right tools. The main program I've heard discussed and the one I use is ProStockMaster and I can't see any others doing a better job really. It's easy to navigate and gets the job done, simple as that really. Download it at http://prostockmaster.com/ and you're on your merry way.
Right, the hard part (well, the "hard" part) - you have to think carefully about the keywords you use to make sure that the right people see and download your images. That's the theory, at least. In reality, there is a wealth of ways you can do this relatively painlessly. One way, which I found very useful at first since it reminds you of words you might have forgotten, is using the PicNiche toolbar with Mozilla Firefox. It may take a little bit of downloading bits and bobs, but should be worth it in the long run. Effectively, you type in the main keyword relating to your image and it gives you a long list of related keywords which you can simply click and then copy to your clipboard. It works a treat, in my experience, so it's definitely worth the download (which is luckily free).
Another (slightly less moral) way of doing it, which I shan't actively condone, is to simply find a similar looking image and simply copy their keywords; check out istock if this appeals to you. However, recent developments at Shutterstock have seen a new key wording tool, which lets you type in your keyword, choose the images that are similar to yours (about five) and they provide you with your list of keywords. It's basically the tool I've always wanted, however sad that may sound...
Whatever you do, don't try and concoct a list of keywords in your head! No matter how hard you try, your list could be twice as long and SS have just made your life so much easier!