Where will you be using your speakers? If the need for portability is a chief concern, boombox-style systems may suit your needs better, since they offer respectable sound quality and a lightweight, easy-to-carry design. Larger (and more expensive) speakers will offer a bigger range of audio clarity, but will be more likely the system that you dock at home and use more as a home stereo system.
What kind of music do you usually listen to? The best reviewers test speakers by playing a variety of musical styles – some may rate better for classical or jazz than others, for example. It's worth combing through reviews if you have a particular preference.
Consider the connection. Make sure the wired or wireless speakers you want will work with your iPod, iPhone or Android device, particularly if you have an older model. Manufacturers typically list this information on their websites.