Any tips for completing collections?

Collection items can take forever to come by. For almost every single collection, I've had all but that one last item for the longest time.

The best advice I can give you is to be patient and wait until you've completed level 3 mastery for the job tier of the collection you're trying to complete. Up until that point, you're not really "wasting" your time farming for collection items.

Be sure you keep your wishlist updated with items you need, in case a generous benefactor in your mafia happens to check it!

If you still need items after you've acheived level 3 mastery, you might want to take a look at what you do have and head to one of the many trading discussion threads: good places to start are the Mafia Wars apps page, the official Zynga Mafia Wars trading forums or the mafia-wars.org Trading Post. If you belong to a clan or have a lot of mafia members who are still in your friends list, you can also advertise trades that way.

Trading can get pretty involved since many people offer collection items in exchange for drop loot (AR-15s, Humvees, Limos, etc.) or for special loot (Blackmail Photos, Untraceable Cell Phones, etc.) I would advise seeking out a 1-to-1 collection item trade (i.e. swap a Ruby Ring for a Sapphire Ring) before you take advantage of these offers... you will need a LOT of special loot to complete jobs later in the game and lots of drop loot if you want to equip a full mafia. See these FAQs for more info on why giving away special loot early on is a bad idea.

Note: There is a glitch out there that causes collections to drop from jobs in other tiers. I consider actively trying to cause this glitch to be exploiting, since Zynga clearly intends for collections to be available from specific job tiers and the accidental discovery of this glitch was originally related to tracking down the cause of the bug. However, it's recently gotten loose into the community. I will not propogate the technique used nor condone actively trying to exploit the bug. Suffice it to say it will not be hard for someone to find through other channels anyhow. If you wind up bugging your job mastery in the process, well... caveat exploiter!