If you get stuck, keep going. Don't frown, curse, stop, or apologise. Simply describe details of sounds, colours, smells, clothes, atmosphere etc. to play for time - this is also a psychological trick because it stimulates your imagination and mental images, and keeps your energy up, which is the best way to trigger your memory.
Or stay silent and still engaged with people's eyes and they'll think it's a dramatic pause, as you let inspiration return (don't look at the floor to remember). Nobody but you knows what you were going to say, so they will never spot your departures from it - there are no 'mistakes'. New improvised details or observations can be gems to keep in for next time.
The site also encourages giving people a real chance to applaud when you're done:
Take time to finish. Look at people, smile, and listen to their applause - do not run away or gesture to dismiss it, the applause is their chance to give you something back, and the instinctive hiding gestures that most people fall into appear as a little insulting. Accept that they liked it!