Not sure about your size? It isn't a bad idea to invest in a soft tape measure - like those used by a tailor. Knowing your measurements will ultimately save you time when shopping because you won't have to go through the hassle of trying things on. If you like to shop online, knowing your measurements is a must.
Buying discount men's clothing online and then having to return it is a pain. Also, take note when you see any clothing that is from Europe - their sizing is different. Any good online clothing store will provide you with a chart that translates European sizes to American - but again, you must be prepared with your own measurements and sizing!