1. Wear clothes that fit your body now. You may have been lighter several months or years ago, or maybe you are about to start a new diet. However, if it doesn't fit you now, don't wear it. This might mean that you have to invest in some new clothes that fit you. Too-tight clothing just draws attention to the fact that you're overweight, emphasizing curves you may not want emphasized. Similarly, over-sized clothes don't make big people look smaller, they just make you look bigger. While Kate Moss might look even more rail thin wearing something that is over-sized, this is not the case with larger people.
2. Don't buy loose-fitting clothes to appear smaller. Chances are they will only make you look sloppy. A T-shirt that is two sizes too big will be too wide in the shoulders and neck and hang down below your crotch, making you look even bigger as a result.
If you have problems keeping shirts tucked in, you can change that. Buy shirts labeled "tall", which are usually available only in large or XL. If you are in that size group, then they will fit you well without being too big. Put your shirt on first, and then your pants. Do not do too much "blousing out" of the shirt, particularly at the front, but also don't tuck too tight--somewhere in between the two extremes will look best.
3. Find clothes that fit the largest part of your body and have them tailored. If the pants fit in the waist, but are busting at the seams at your thighs, buy a larger size that fits your thighs and have them taken in at the waist. It is better to pay a little extra for pants that fit you perfectly than $50 for pants that don't.
4. Dress for the occasion. Some overweight men may not want to wear shorts or t-shirts in the summer because of insecurities. However, dressing in out of season clothing to try and cover it up will only draw more attention to you. Wear clothing that suits the environment to fit in instead of sticking out.