My mother's trick, which I've heard about from others too, is to stir a knob of butter into the cooked jam. Don't ask me about the science behind it; supposedly the butter helps to clarify the jam and to get rid of the foam as it cools. My mother's jam is always crystal clear and foam-free, so I don't mess with the recipe. —Merrill Stubbs
Combine the strawberries, sugar, salt and lemon juice in a medium, heavy saucepan. Set the pan over low heat and cook, stirring frequently, until the sugar dissolves and the mixture starts to bubble.
Continue to cook over low heat for about 30 minutes, until a bit of the jam sets on the plate you've been keeping in the freezer (when you tip the plate, the jam should run only very slowly).
Turn off the heat and stir the butter into the jam—this will help to clarify the jam and get rid of the foam once it cools.
Spoon the hot jam carefully into hot sterilized jars and either process the jars or seal and keep refrigerated. If refrigerating, use the jam within a week or two.
Some sterilizing notes:
Use glass jars with no chips or cracks and tight-fitting lids.
To sterilize, wash both the jars and lids thoroughly with hot, soapy water. Bring a large pot of water to a boil and boil the jars and lids (don't boil metal lids or rubber seals) for 15 minutes. Alternately, after you've cleaned the jars and lids, arrange them (lids open sides up), without touching, on a baking sheet and put them in a 175° F oven for 30 minutes. To sterilize metal lids and/or rubber seals, put them in a bowl, pour boiling water over them to cover and let them soak for a few minutes.
Sterilize all of your additional equipment (tongs for handling hot jars, funnels, ladles, etc.) by dipping them in boiling water for a few minutes.