http://fieggen.com/shoelace/secureknot.htm will hold your laces ties where a standard knot will work itself undone. This is particularly useful for round, synthetic laces.
>>Julian+(OP)
I use this knot once every few years - when I get new shoes or new laces. This knot is so amazingly strong that I only ever tie it once, then I just slip my shoes on and off.