Particularly if you have small, loose items, use ziplock bags all the time.

• Get a few different sizes.
• Pack multiple smaller sets of bags into larger gallon-sized bags.
• Never try to squeeze stuff into one; always use a bag that is bigger than its contents.
• Other uses for ziplock bags: trash, wet items (I put my travel sized umbrella in one so that when I put it in my bag after use, it won’t get my other stuff wet; good for toiletries too), food.
• Good for electronics because they create a mildly water resistant cover. If you really care about your stuff though and plan on getting wet, get a dry sack.