Eat lots of produce, especially vegetables!
The Paleolithic diet contained about 10-12 portions of produce per day, with a portion being around a half cup. The total amount is about the size of a decent lunch salad! This is similar to what the American Cancer Society suggests as a cancer-prevention diet. The 1/4 of people who eat the largest amount of produce have roughly 1/2 of the cancer rate as those 1/4 of people who eat the lowest amount of produce. Basically, a person can reduce cancer risk by 50% simply by eating vegetables and some fruits, in relatively small amounts.
Produce is loaded with various anti-oxidants such as flavenoids and pigments, as well as fiber, vitamins and minerals. The potency of these anti-oxidants is affected by time and storage, which tends to decrease by oxidation. This means that produce is best eaten as soon after picking as possible. In addition, this means that long distance transport sharply decreases food values.
The nutrients in produce are not particularly affected by cooking. In fact, cooking breaks down some of the cellulose that we have no enzymes to digest, making the vitamin and mineral content of the vegetables much more available. This is even more important for those with weak digestive systems. For this reason, we recommend that vegetables be eaten mainly in the cooked form, with raw foods as a condiment for the most part.
Raw vegetables such as salads can be digested more readily if tossed with vinaigrette dressing. Make your own dressing with equal parts of good olive oil and good balsamic vinegar seasoned with a touch of umeboshi plum vinegar and other herbs to taste. This basic dressing may be changed in many ways such as adding Dijon mustard, minced scallions or red onion, blending in fresh basil, etc. Consider marinating steamed vegetables such as green beans, broccoli, zucchini, carrots, etc in a tasty dressing.
These days, most people do not eat nearly enough vegetables, and instead have replaced vegetables with starch such as bread, rice, pasta, and potatoes. Consider the content of most American snack foods. This is a real problem.
Fruits during Paleolithic times were not the huge, sugar-loaded things that they are today. They were very small and somewhat tart. The best approximation of Paleolithic fruit we have today are berries. Berries have a lot of antioxidants and other nutrients for the amount of sugar that they contain. Sweeter fruits should be eaten in more limited quantities.
Anyone who does not eat 10+ servings of veggies and fruit a day should consider supplementing their diet with some sort of Green drink (i.e. vegetable/grass). I personally like the GreensFirst or PaleoGreens supplements. They not only taste great but are extremely nutritious and organic!