I use Endura Perfect Tear Lay Flat. It looks like a giant roll of masking tape. I have found that it works the best for removing vinyl from the backing easily, and then also releases easily when the vinyl is placed in its final destination. The drawback to this tape is that it is not clear so you can't easily see where you are positioning the vinyl.
I have also used Clear Choice AT-60. It is more plastic-like. The big positive to this type is that it is clear so you can easily see when you are positioning things. It doesn't seem to pick up from the vinyl backing as easily as the Endura tape.
I keep both around, but prefer the Endura brand whenever possible. I ordered both from sign warehouse.
http://www.signwarehouse.com/shop/merch ... A-EMB-PTLF - Endura Brand
http://www.signwarehouse.com/shop/merch ... A-EMB-PTLF - AT-60 Lay Flat
Hope this helps!