My One Plant Rule
My one plant rule to put it simply is that I never buy more than one of a kind of plant if I think I can root it easily. If there is a plant I like and it's at regular price it's OK to buy it as long as I limit myself one of that kind of plant. With that one plant I'll take cuttings and make several more plants from the original plant. Most perennials are very easily propagated like the salvia I mentioned last week and annuals like coleus can easily be propagated in water to make more of once they are mature enough.
The Tomato Rule
Now I usually start my tomatoes from seed but if I did buy them from a store I would follow my one plant rule for tomatoes. If you take a stem of a tomato and place it in a jar of water it will root - very easily. Just leave one or two leaves at the top and remove all the rest and watch for roots to grow. It's a very easy way get a deal on tomatoes!
The Basil Rule
The basil rule is pretty much the same as the tomato rule. Basil roots very easily in water. Last fall I preserved basil over the winter that is still alive and sitting on our windowsill. It's desperately wanting to be planted outside so in the next few days I'll see that it gets a home in the garden.
The Multiple Plants Per Pot Rule
I always look for the pots that have more than one plant. Often growers will sell plants like basil with three seeds initially planted in the pot to increase the success rate on their pots. Having three seeds in a pot is a little insurance against losing one or even two plants. Sometimes perennials and shrubs will have layered or are able to be divided. In either case I look for pots I can get a two for one deal on (or more if possible)! Ornamental grasses are a prime example of a plant that can be divided into several more.
The Six Pack Rule
Annuals typically are cheap - cheap - cheap when found in six packs but sometimes stores will sell a single larger version of the same annual for a higher price. Not only is the plant more expensive but you one get one plant! Those little bitty six pack plants will easily grow to be the same size as the larger single one and may only take a week or two longer. It just makes more sense to buy small.
The Already Rooted Plant Rule
I realize that this falls mostly under the first rule here with plant propagation but it is a slightly different variation. Sometimes you'll find plants that have a little bit of rooting already happening. I bought a lobelia the other day that had this nifty trait happening. The plant had two main stems and one of them had already sprouted some roots along the stem. This happens with many plants when the stem is shrouded in darkness from the surround foliage. The plant thinks those areas are under soil and sends out roots to gather more water. When a plant roots along the stem it's a perfect opportunity pick up an already rooted cutting! Just snip it and pot it up. It may need a little babying until it's roots are extensive enough to fend for themselves. Looking for this trait is also a great way to figure out plants that are easy to root.