Pack your suitcase with only the necessary items. When packing your suitcase remember this. "Less is more." If you pack basically everything you own thinking you will need it you will be adding too much weight to your bag. You also want to lessen the chances of losing stuff while on vacation.
Only pack what you'll absolutely need. If you're going to the beach or will be staying in a hotel with a pool pack two bathing suits, sunscreen, 1-2 towels, a hat, and sunglasses. If you are going somewhere for just a weekend pack 3 shirts, 3 pairs of pants/shorts, 4 bras (if you're a girl), 4-5 pairs of underwear, and pads (once again- if you're a dude this does not apply to you). If you're going somewhere for a week maybe double this amount. If you're going somewhere for a month or more do the math.