Adding an alarm is a MUST. It also keeps away kids that will scratch your paint!
Bring a chain, esp. if you park your bike to the same place, leave the chain there, attached to some unmovable post. To protect against thieves cutting the chain, make sure the chain does NOT touch the ground when it's wrapped to your rim. (they cannot get leverage to use a chain-cutter).
This protects you against a truckload of people coming and lifting your bike to the truck (happens often!)
Park your bike where it will be visible (i.e. under a streetlamp). they will think twice before fiddling with a bike with a blaring alarm where there are others that can see it.
If you use a disc lock, attach a WIRE to it, and route the wire to your handlebars.. It's very easy to forget a disc lock and start the bike ("clunk!"). You'll see the wire and remember the lock. It also deters the thieves thinking it's a fancy alarm