I would recommend covering them with a plastic bag or something to keep the bees from getting to them. That is the sure solution on keeping them from cross-pollinating. Maybe place a thing of honey or syrup away from the plants to keep the smaller insects from cross-pollinating, also.

When blossoms appear, shake the plant for 10 seconds. That should successfully pollinate them. If you see problems with some of the fruit appearing, shake the plant for an extra 10 seconds next pollination.

Heirloom tomatoes have perfect flowers (both male and female pats) and self-pollinate most of the time, without any special precautions. If you want to be ABSOLUTELY positive that they do not cross pollinate, then bag the blossoms before they open and leave them bagged until the fruit forms. Then you would need to somehow mark that fruit when you took the bag off so that you would remember which one to save seeds from. I saved seeds from 3 different varities last year and do not think that I had cross pollination problems (the PL came up PL, so I know those are pure). This year I have about 40 varities though.....
Check out the FAQ over on the tomato forum for lots more useful info on saving seeds and cross pollination

Unless you live on a very large peice of property bees will easily fly from one tomato plant to another no matter where you put them. These are tiny sweat bees and large bumble bees,looking for pollen to feed their young so honey/syrup will not attract them away from the plants. Use tulle/organza mesh bags -- 5x7 drawstring sachet bags sold at Walmart for weddings, not plastic. You can also buy the fabric and make your own.

I cut the feet off torn nylon stocking instead of buying in bags.

I was given some heirloom tomato plants last year. The tomatoes were tiny, size of blueberries. I saved the seeds and have seedlings. Is it possible to plant a tomato with large fruit near by to get larger heirlooms with cross pollination?