Do your major swag sweeps on the first day of a show. It's usually the busiest, and the time when exhibitors want to make the biggest splash with special giveaways and promotions. Make the first item on your swag list a swag bag. You'll collect more in the long run if you don't have to carry it all in two hands. Look for crowds huddled around a small table.
People tend to migrate towards free stuff.
Bigger booths don't always have the best stuff. They just usually do. This is also true for products on exhibit. Find other people who have cool stuff, and look for the logos. If a company's smart, it'll have its name plastered in bold across the front of its freebies.