Masking

Tamiya as everyone says, but you can use blue painters tape to fill in big areas. I've great success with Frog tape - they have one formulated just for acrylic paints.

mitsdude

I don't understand why Tamiya doesn't have tape more narrow than 6mm.

They do have 3mm, but the Pactra stuff is still half that width.

There's a brand of masking tape called "Duck," available at Wal-Mart, which is low-tack, and cuts nicely into very narrow strips when you need them.  

Very cheap and haven't had it leave any residue, or lift paint when the surface is prepared well; but then, I use enamels, not acrylics.

I recently masked the rotor blades of an apache with Tamiya tape at the blade root (near the center) and the cheaper hardware store stuff for the large area.  I wrapped in a spiral pattern and found when I unmasked that it left behind sticky residue along the edges.  I found that goo b gone took the residue right off without causing any problems with the paint.

I've also found that for large areas, it's tedious, but clear scotch tape does well for the center areas and doesn't pull up paint.

Groot

I don't understand why Tamiya doesn't have tape more narrow than 6mm.

I use Tamiya for small areas, or for the edge of large areas, filling in the rest of the area with hardware store stuff.  The Tamiya makes the smoothest edges and prevents bleed under the best (though sealing is still a good idea) and does not pull up previous layers of paint.

I do keep a roll of Pactra's 1/16 inch stuff because it is narrower than any Tamiya size, but it is not as good a tape.

Well, I actually use a combo of tapes just to experiment and find the cheaper tapes to work better at masking canopies, having a stronger tack that won't lift up on curved surfaces after a few minutes. Then I use either the good Scotch  blue tape or Shuretape for painted areas. Shuretape is similar to Tamiya's but much cheaper. It's available at Sherwin Williams.

Thank you for your input everyone

I usually avoid the cheap brown masking tape. I dont like to use it even when I'm painting walls in the house. For my models I like Frog Tape for larger areas, but blue painter's tape usually works just as well. I can cut it for smaller areas, but it can gets a little more time consuming that way. Tamiya tape might be the better option for small areas and details. I've not used it, but it's on my list of supplies to get.

If you're brush painting, it's hard to avoid paint building up on the edges, regardless of the tape you use. I've had that problem. Spraying helps, but as Jon_a_its said, you'll want to build up the color with multiple thin coats. Two coats usually gets the job done for me, sometimes three. Just depends on what you're working with.

Tamiya for the edges,but use your cheap stuff for filling in the big areas

Tamiya Masking tape is regarded as the 'go to' option.

If you are getting tram lines & build-up, you are allowing paint to build up on the edge of the tape, & it will 'wick up' the edge of the tape. The best way to avoid this is to spray thin coats to build up the colour, and spray from the masked edge to where you want the paint.

You don't say if you are spraying or brushing. paint buildup is a lot harder to avoid with a brush.

Google  Masking Tape Tips for Models for err tips