Weed - This one is pretty obvious but it has to be done! Weeding the garden removes competition for valuable nutrients, opens up the area to better air flow, and gives your plants some room to grow. Take the weeds and drop them in your compost bin to break down. Weeds have a knock for growing in places where other plants don't like. That means they are able to pull nutrients from those areas into themselves. When you compost those weeds (hopefully before they have gone to seed) the nutrients end up in your compost which then ends up in your garden. 
Soil Test - I've gotten by in my garden without a soil test so far but it is a valuable tool. When plants don't grow right (or at all) there may be something wrong with the soil that a soil test will help to figure out. Soil tests can indicate nutrient deficiencies or an improper balance of pH. When in doubt though it's not a bad idea to add compost...Add compost - Compost adds valuable organic matter to the bed. It creates an efficient environment for microbes to convert minerals and nutrients into usable forms for plants to use. It also helps the soil keep a good balance of moisture available to your plants. I've said this before but if you do nothing else for your plants add compost!
Amend - If you need to amend your soil with anything other than compost do it before planting to ensure that it gets mixed evenly into the soil. Bonemeal, bloodmeal, alfafa meal, cotton seed meal, and any other organic amendment can be better available to your plants if mixed into the first few inches of soil before you plant. An even distribution of nutrients allows the plants to grow evenly. If you only put the amendments in the planting hole the plant's roots will want to stay in the hole - which is not what you want. Large root systems make better plants!
Arrange Irrigation Lines - irrigation is best planned out before you plant but I generally tend to do it afterward. If you know where all your plants are being planted go ahead and plan our your drip lines and hoses. It's easier to work around the hoses than it is around the plants!
What do you do before planting your vegetable garden?