To keep pests down and increase yields, it is a good idea to divide your plot into four or more sections and group your crops into types- peas, potatoes, roots, cabbage, onions. Each year, grow a different type of crop on each section. Pests then have to wait four or more years for their food to reappear on that patch of land.
More on crop rotation from Garden Organic here
Dealing with slugs
Slugs like long grass and rotting vegetation. Keep the grass short and remove piles of weeds to help keep slug numbers down. Hunt them down: preferably during or shortly after rain, or in the evening.
Trap them: use beer to attract them from a wide area, or give them somewhere to hide, such as upturned citrus skins, piles of weeds/damp newspaper or wooden planks, then collect them from the traps and dispose of them using your preferred method. Infect them: water the soil with parasitic nematodes, effective for 4-6 weeks. If all else fails, poison them using Wildlife-friendly ferric phosphate pellets.
Dealing with drought
Water well on planting, Mulching, weeding, occasional soaking encourages deep root growth and is more affective than daily watering.