I bought one o' these to help eliminate drips from the brush shaft ... it works great, but be sure to rub a little vaseline inside the cap threads to prevent them getting cemented in place! http://springfieldleather.com/16721/Jar%2CPlastic%2CCement%2C32oz/
Solder flux brushes are also excellent for use with contact cement, and they're inexpensive and disposable! ( you can buy 100 for about $20 if you shop )
Nuttish has the right idea too .. blue tape works great for masking, but do be careful about what type you use. Not all of them have the same amount of tack for the adhesive .. you want very low tack for the grain side, and even then do not leave it on longer than necessary.
Not necessarily part of the original post problem ... but cool in any case ... If you need to contact cement a large area, or long line ... glue up both parts, twice if necessary, let them get tacky and then put a big piece of thickish plastic or a piece of waxed paper between the parts and slowly pull it out as you align the pieces!
Bill