You can separate off any small bulbs that form around the main bulbs. Be careful when digging them up (in summer) as some of the babies may just drop off as you do it. If there are no bulblets then you can cut small notches into the basal plate (the hard part at the bottom of the bulb where the roots originate) and replant. This will produce bulblets for next year for you to divide off. Or if the tulip is a species type, you can collect seed (don't deadhead the flower!!).