I like dry bags, like the WXtex by Pacific Outdoor Equipment.

To store bulky items, simply fold the items and insert them into the bag, then, on a flat surface, roll them up from the bottom to the top. Then, while the item is rolled (helps to put your knees on the roll), close the top. In the case of the WX bags, you have an extra option since they have an air valve.

Simple and effective.

If you are worried about them self inflating over time, you might go to a heavier duty version, maybe even "rubberized" ones intended for more serious water use:

sample

-john