You can trill just by moving your mouth

Piano players might be familiar with trilling, which is playing two adjacent keys ("D E D E D E D E") over and over. You can do the same with a trombone. The only difference is you don't use your fingers. Instead, you use your lips to trill. Leave the slide in the same position, and produce a sound as you quickly switch back and forth between blowing methods.

Example of lip trilling