Yes, Silly Putty, that lump of clay like thing that you played with as a kid. Turns out it's a great tool for doing masking, especially for camo. I have heard a few times that people complained about it leaving a residual, but I have never experienced it. Of course, I use only acrylic paint, so perhaps it's a reaction with enamal paint? I have left it on for days with no issue - just make sure you let the first coat of paint fully try first. Here's a picture of me using it for Tamiya's JSDF main battle tank over Tamiya acrylic paint.
Tools and Accessories
Grex 1/8 HP Airbrush Compressor (AC-1810A)
You have an airbrush? You need a compressor - using canned air is ok to learn on, but they get expensive real fast. So invest in a good compressor - but the question will be which one and what is your budget. This GREX compressor makes a great first compressor with a relatively cheap asking price of $199 MSRP.
Airbrush holders and spliter.
So where do you store your airbrush? I have two, do I have to swap between the two? Where do you put your brush during a painting session? I use an integrated cleaning pot and holder like the one on the left.
Cordless Paint Mixer
Sometimes having a simple tool for the job just makes life easier - and so is the case for an electric cordless paint mixer.
Vallejo Weathering Effects
Masking Tapes for painting

Masking tape is one of the essential tools used in modeling - but not just any cheap tape you pick up at the big box stores. Here's a rundown of the types I've find most useful for paint masking.