Something my family has done forever is to re-use gallon jugs as little green-houses. If you come across any of those semi-clear gallon milk or water jugs, dig em out of the dumpster and put them to use.

First, plant your seed or seedling wherever you want it to permanantly grow. Take a gallon jug and cut the bottom out of it. Now, cut a little wedge into it, so water can enter/escape as it pleases. Next, place the jug over the seed/seedling. Make sure the jug's bottom goes a couple of inches into the soil so it won't blow away when a gust of wind blows. You should be able to see the wedge you cut into the jug. If you are using a drip system, you should establish it before you place the jug over the seedling. Otherwise, water as usual; the notch will let in plenty of water. This works especially well with tomatoes.

When the plant starts trying to come up out of the hole on top of the jug, then its time to cut the top out. Once the plant grows over the top of the jug, it's ready to remove, and replace with a tomato cage.