Rule 1: Absolutely NEVER should you kill or take a live animal with you. This includes starfish, crabs, fish, turtles, etc. If it's already dead, then go ahead. Also, be aware of your state's endangered species restrictions. If you're not sure what it is, then don't mess with it. This includes some plants. THIS is why you brought your camera along.
Rule 2: Exercise caution! Please be aware of wild life, hazardous trash,slippery surfaces, weather, the sea condition, etc...
Rule 3: If you're going to take something, please make sure that object isn't someone's home. You'd be surprised at what crabs like to make their little houses out of. I've even seen them use plastic sports bottle caps. (I did NOT have my camera on me, sadly.)
Rule 4: Do you remember that plastic grocery bag I mentioned? While you're digging for treasure, you will most likely come across some trash. If you simply pick up whatever garbage you find, you'll be leaving that beach more beautiful and safe than it was when you got there. What a nice feeling!
Ideal conditions for beach combing are as follows:
during the winter
early in the morning
a low or receding tide
right after a storm
I know it's not always possible to combine all 4 ideal conditions. The most important of these guidelines is that you get there first. It's like going to the flea market. If you get there before everyone else, you've got the best selection. If a storm happened to pass the night before, it probably churned up a lot of interesting and rarer things for you to see. During low tide, there is much more sand to scour, plus you've got a "fresh" selection to choose from. When the weather is least desirable in the winter months, you may happen upon a spot that hasn't been perused by humans in days or weeks.
Safety Reminder: When beach combing after a storm, use caution and common sense. If the ocean looks rough, don't go anywhere near the water. Even if you think you're keeping a safe distance, you never know when a rogue wave can come along and knock you into some rocks, or worse, drag you in. Please check your local weather forecast for the sea condition if you're unsure. I know that if you live near the ocean, you probably know this already, but for the tourists, I want to make it clear that the ocean is extremely powerful.
The rest is all up to you. It's preferable that you "take only pictures and leave only foot prints", but so long as you abide by "the rules" and come prepared, you're in good shape.