Potatoes

You can mound the rows up now, or cover the leaves as they grow - both methods seem to work ok.

If you leave the ground flat the soil at potato level gets more warmth from the sun. As they emerge pull the soil over them bit by bit, first to protect from frost, and later to prevent green potatoes and destroy germinating weeds.

If you earth them up too much when you plant the potatoes can break out of the side of the ridge as they will always take the shortest route to the light.

I second Salmo's way of little and often.

I'm in the 'little and often' camp (but not so often that you run out of soil to earth up with just before your last frost warning of the year and have to go on a mad scramble to find your fleece and some pegs!  )

The leaves are the source of the starch for the tubers, so it makes sense to me to get the leaves to the light ASAP, which means not planting excessively deeply or earthing up too early or by too much, although it is of course essential to keep them protected from frost for as long as that remains a risk.

Earthing up isn't just about avoiding frost damage and green potatoes - much of the crop comes from shoots produced from the underground stems, so the original seed spuds need to finish up quite deep even if they weren't originally planted very deeply.

My method is to dig a trench and mound the soil up alongside. Plant the potatoes in the trench and cover with 2 or 3" of soil. As the foliage comes through you can easily pull the mounded up soil over it - carry on until the trenches become the mounds. This not only makes earthing up easier, but has the advantage that rain can more easily reach the tubers in the early stages.