Milkrun: noun, spacer slang for a highly profitable, but tedious, trade route. Also; milking: verb, doing a milkrun repeatedly.
For example, in FE2 and FFE, the route Sol - Barnard's Star is the mother of all milkruns. It is very short distance, it is absolutely safe, and guarantees a good profit for every trip. You don't need any weapons or equipment other than Autopilot. Doing this route will put a spacer in the position to buy any ship they want within a matter of months.
Take luxury goods from Sol to Barnards Star and computers or robots back to Sol. Computers and robots are usually sold out in Sol so check the bulletin boards on the three orbital stations around Earth and the one around the moon for anyone willing to pay double. This makes it more profitable and a little more exciting.
Van Maanens Star is also safe once you have a permit and know at least one merchant. Smuggle Luxury Goods and Liquor from the Sol system and return with computers.