Like Goddesss says, if you're not dry-cleaning:
1) hand wash with gentle detergent
2) air dry properly
3) iron properly (starch if necessary)
4) buy good quality
...buying quality is essential if you want your clothes to look good after washing. Cheap knits like t-shirts especially can almost be relied upon to lose their shape after one wash. In fact I try and buy clothes that look even better after repeated washing and use...these need not be expensive if you know what you're looking for. (Petit Bateau t-shirts are a good example.) Ironing is often overlooked but can really make your clothes look better. Read the care label too. Taking care of clothes is really a complex skill, so study, try, and practice! Good luck.
