I like to keep a tape measure, a calculator, pencil, paper, and some work gloves in the truck because you really can't forget those.  You should also bring a shopping list that details what you will need on your next project.  It takes a lot of time to figure out what you need before you go shopping but it's well worth the effort.

A hardwood store is filled with guys who work for a living.  They typically aren't *really* concerned with what you buy but they do care about what you do.  They are also the guys who will calculate your board feet so it's a good idea to get on their good side (more on that later).  While you're shopping be sure to leave the stack of lumber in perfect condition.  If the stack was a mess before you got there leave it in perfect condition anyway.

Also, be yourself.  There is nothing a blue collar guy respects less than an engineer acting macho while buying figured maple on a Saturday.  You don't have to be tough to be likeable in the eyes of the staff, you just have to be nice and you have to leave the stacks of wood in good order.

You're unlikely to find boards in the exact dimension that you're looking for.  It usually takes me a few hours to figure out which boards will be used for each part of the project.  As I figure it out I make little notes on the lumber in pencil and set it aside.  Some people spend all day picking out their lumber so don't feel rushed.

Calculating board feet is dead simple.  Multiple length * width * thickness then divide that by 144.  For example, if you buy 8/4 (pronounced "eight quarter" and it means 8/4" or 2") thick stock that is 6" wide and 8' long then you have (2 * 6 * 96)/144 = 8 board feet.  Now, it's rare that wood is exactly 6" wide.  It's more likely to be 6.25" wide.  If you're a good guy and you keep the pile of lumber in good shape the staff  there is  likely to round it down for you.  If the wood you've chosen has a wane (rounded edge with bark) then they need to do something other than charge you for the full width.  If you're the kind of customer they love to see then you're going to get a better price.  I always calculate the board ft. myself but most of the time the guys there give me a better deal than I come up with on my own.

Some guys also like to bring a hand plane and spray bottle filled with water.  If you plane a smooth section of rough lumber and spray it with water you'll really get a good idea of what the wood will look like when it's finished.  This is a good idea if you're buying rough sawn figured lumber.  However, most figured lumber is already planed so you don't need to plane it yourself.  Also, I don't like to spray the wood with water, I can imagine what it's going to look like on my own.

I've added this part because at least 5 people have asked me where I buy lumber.  I've shopped at most of the local vendors and nobody is perfect.  If I need a broad selection I go to The Hardwood Store, if I want a good price and a short drive I choose Robbins Custom Sawing.

The Hardwood Store of North Carolina

Pros:
Competitive pricing
Quality lumber
Good Selection
Nice staff
You can pick through the stacks, they'll even help you if you want
They are open on Saturdays
Nice dry lumber storage

Cons:
Gibsonville is too far away

Bottom line: A good vendor for times when you're buying a lot of stock.
 

Capital City Lumber

Pros:
Location
You can pick through their stock
Open on Saturdays

Cons:
Prices are high
Selection is a bit weak
The staff is overbearing
The lumber is practically stored outdoors (large barn with open door) so it tends to move, twist, bow, etc. when I bring it into my shop

Bottom line: I rarely come back here.  I'd rather buy at Woodcraft in Raleigh for a few dollars more than suffer with their wet stock.

 

Wall Lumber

This is the largest hardwood store I've seen in the area.

Pros:
Good prices
Quality Lumber
Good Selection
Nice Staff

Cons:
Lumber storage can be a little moist
You don't normally pick out your own lumber here but with a little charm they will let you.
It's too far away
 

Bottom line: This is a pretty good vendor but I usually only buy here if The Hardwood Store doesn't have what I want in stock because they are pretty far from the Triangle Area.
 

Robbins Custom Sawing and Lumber

Mike Robbins is a good guy with a woodmizer.  He sells wood that can be found locally and he is a pleasure to deal with.
Shop: 919.779.0719
Mobile: 919.270.1569
Here is his address: Mike Robbins - 2200 Bull Run Drive, Apex, NC  27502
 

Pros:
Short drive
Good guy
Best prices in the area
Quality lumber (well dried, stored, etc.)
Lots of wide boards (can be used for rustic furniture or other interesting designs)
Often marks out book matched stock

Cons:
Limited selection (call ahead)
Not a retail outfit so you buy by appointment

When I asked him what he sells he gave me this list over the phone:

Bottom line: This is my first choice lumber supplier when I'm buying lumber that he carries.