1. Flavor. The key to big flavor is cooking the jam as quickly as possible, and trying to cool it quickly as well. Think of it as "blanching" the fruit in sugar.
2. Consistency. The first time I attempted strawberry jam, I waited until the hot mixture looked "jammy." Once it cooled the result was strawberry candy that hardened together in a sticky mess. The next time I was overcautious and got strawberry pancake syrup (not a terrible thing, but still not jam). The trick is to wait until the mixture looks like melted jam. It sounds silly, but it was an epiphany for me. A more specific tip is stirring the mixture with a metal spoon, and if you can see the bottom of the pan as you stir, you're on the right track. It should cling to the spoon and slowly drip off as well.

Pictured: White Peach/White Nectarine, Seascape Strawberry, Mixed Berry (Red and Blonde Raspberries, Blueberries, Blackberries)
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Ingredients:
3 lbs of fruit (ie 3 baskets of strawberries, 6 half pints of raspberries, etc.)
3 cups of sugar (this is for sweet strawberry jam, a little more for tart fruit (raspberries), a little less for sweet fruit (peaches)
Juice of 1 lemon
3 tablespoons of liquor (optional) I used chambord for the mixed berry jam and grand marnier for the strawberries.
Directions:
Cut the fruit into the size you’d like it to be on toast. For strawberries I quarter them if they are on the smaller side, for berries they will fall apart so no worries about the cutting, for peaches halve and slice very thinly)
Put the fruit into a large bowl and cover with the sugar, lemon juice, and alcohol (optional). Cover it with a kitchen towel and let it macerate in the refrigerator over night. The sugar is acting like salt does in the preservation of meat; it will cause the fruit to leech out its liquid, creating a fruit-sugar syrup.
On the second day, pour the syrup into a pan and heat until it bubbles (about 5 minutes on med-high) and then add the fruit. Stir until the entire mixture is bubbling (about 5 more minutes) and return to the bowl and refrigerator for another night.
On the third day, put the entire mixture in a pan and let it start bubbling over medium heat. Stir frequently so no part is boiling.
Meanwhile, boil you jars and their tops in water for at least 10 minutes, then allow them to dry out on a clean kitchen towel.
When the mixture is ready (this is after about 10 minutes of bubbling, with the mixture clinging to the spoon—DO NOT OVER COOK) take it off the heat and ladle the jam jars until almost full, leaving about a ½ inch open at the top. Secure the tops to the jars and clean off any excess jam, and then place on a plate, upside down. The plate is just in case some jam leaks out. Place the plate of jars in the refrigerator to cool over night.
The next morning, you will have a perfect partner for toast!
If you like the jars I used (i love their voluptuous shape) you can get them at Weck Canning at a very reasonable price.