Promotional products really allow people to see your brand and remember you. If you hand out a your branded promotional product, when they go to use it, they will instantly be reminded of your business, and your products or services.
I make sure promotional product the suits the niche of your business, who you are targeting and is consistent with your overall brand strategy. Say you want to reach a tech company - an item like the branded USB stick I created for my IT client worked out wonderfully. I make sure the promotional product I design is useful and subtle, as it will be something the potential customer might use all the time, but without them feeling like you are shoving your business down their throat.