Several methods. Rubber bands outside will degrade quickly, but black twist ties around the outer ties should work. Simple cheap solid-strand wire (something like 20 gauge or so) wrapped around the end tie on each piece of track would probably work too. For something more permanent, Split-Jaw clamps are probably the best.
You don't mention if your track has solid rail (like LGB, Aristocraft, USA Trains) or if it is hollow (old Bachmann plated track, old Lionel Track, etc). If it is hollow, I would not recommend using split jaw clamps as they might crush it. (Then again, I wouldn't recommend either of those brands of track for outdoor service anyway).
