Use one to three words separated by a hyphen. (hyphens are easier to remember and faster to type than underscores) You will have more success if these are real words than gobbledygook, and even more if the words bear a remote resemblance to the topic of your lens.

People don’t really type URLs into address bars anymore (excepting root domains) so, while being memorable as a lens url is useful (like /tricks /squidconomy /basichtml and the like) it’s better to be bookmarkable.

Do you think it’s a coincidence that bookmarkable and remarkable are very similar? Probably not says the pirate in me.