If you want to secure a large solid item, bungees, tiedowns, straps, and rope will serve just fine, but for a plethora of little junk, go with a cargo net. Cheap nets sell for as low as $10, and you can pick up a reasonably good net like the one pictured above for as little as $20. If you want quality and time-saving features, look for a net with adjustable straps across the middle.
And if you’re in the mood for overkill, you can get a steel net rated at 10,000 lbs. for $500.