Some things to keep in mind when canning:
Follow the recipe. It just works out better that way.
Watch your time. Don't cook too long. Don't cook too little. Cooked too long, and flavor suffers. Cooked too little, and texture is too liquid. When pressed, I err on the side of undercooked; you can always cook it more, but once it's cooked too much, you can't go back. It is an art, one I'm trying to grasp.
Basically, be patient. Don't jump the gun - some jams take half an hour to cook; butters can take an hour or so.
Add lemon. It brightens anything right up! Especially strawberries. More is better, in the case of jams and preserves.
Taste before adding sweetener. Then sweeten to taste. Fruit may be plenty sweet already. If a recipe calls for more sugar than produce, I forget it and look for a better one.
For butters: make sure they're thick before jarring up; it's kind of sad going through all that trouble but then having a seepy, weepy spread.
For that matter, if you ever cook anything in your entire life, make it this strawberry jam.
This is the first canned thing I made. It is an essential. Think of how many batches you might want to have on hand, and then double that amount. Make as much as possible (as many batches as possible). Because, believe me, this jam goes quick. Everybody raves about it, and jars of it are perfect as favors & gifts.
A single teaspoon carries with it the sheer essence of summer; the scent of perfectly ripe strawberries mingling with the warm sun. The smallest drop is opaque; a sure sign of its super-concentrated flavor -- and, of course, its nutrition.
This is technically a "preserve", meaning it's made up of pieces of fruit and thick syrup, but only technically; I like the sound of jam better, so I call it jam. It's slightly runny compared to the average jam, but the flavor is incredible. I'd pick runny and delicious over tough and bland any day.
Originally, this recipe came from Putting It Up with Honey, a gorgeous book by Susan Geiskopf that, for some crazy reason, is out of print.
The process is a bit involved, but so worth it. Just wait til you taste it. You cannot go wrong with this jam.