I often make salads with lettuce and a few vegetables (perhaps some of carrot, celery, bell pepper, cucumber, tomato, green beans, snap peas). I can plan the ingredients before I go to the grocery store or farmer's market, but are there good ways to pick the ingredients once I get there, based on freshness or value? How?
My advice is simple -- don't plan ahead.
Being fresh produce, most of us have no idea before you get to the store what the current stuff coming out of the local fields are.
This is going to affect both price (in season stuff that hasn't been shipped from the opposite hemisphere is typically cheaper, especially w/ today's fuel prices), and quality (how long it's been sitting around, maybe there's been too much rain or not enough, etc).
And, don't forget that you don't need to limit yourself to vegetables in salads -- sliced apples or pears, orange or other citrus segments, etc, are great additions, particularly as we get into the fall and the spring & summer vegetables aren't locally available.
If it's the middle of winter and nothing's fresh, consider jarred marinated vegetables like olives, sun dried tomatoes, peppers, cauliflower, etc.
Well, just pick what looks fresh. Usually when I go shop for groceries, I don't have much of a list, but rather pick what looks good at the market and then let the produce inspire me and make up something either at the market or at home.
Look for bruises and feel the veggies. Vegetables that have been out in the sun for half a day can become soft – especially carrots, parsnips, leeks and lettuce for example. And with lettuce or vegetables like cabbage that are in the form of a head, try to determine how many of the outer leaves you need to remove – with brussel sprouts, sometimes you end up with half the size when you've removed all the ugly leaves.
If you find bell peppers that seem to have seen their best day, try to get a lower price, buy loads and roast them. Delicious in salads.