This article is from the Unicycling FAQ, by Beirne Konarski with numerous contributions by others.
Use a rubber hammer or regular hammer and a block of wood to pound the
cotterless cranks on firmly. Place the opposite crankarm on a (another)
block of wood on the floor to avoid pounding stress to the spokes and wheel,
etc. However, do not pound real hard or the crankarms and possibly even the
axle could be damaged.
After the above, snug up the nuts real tight. Your should never have to
worry about loose crankarms again, WITHOUT using lock tight. The
non-permanent lock tight couldn't hurt though, but be sure the crankarms are
on very, very tight before using it!