There are three things that every collector should do to maximize the return that they receive for their stamps when they go to sell, and I will address them in ascending order of importance. First, it is very important to have some figures on your stamp collection. Do you have records of how much you spent? If so, add them up. If not, make a fair estimate based on the amount you felt like you spent per year over the years, making allowance for the years that your collecting was dormant. Most collectors won't get back the full amount that they spent, but at least this will give you some basis to judge how much your collection is worth. If you can catalog the collection by a recent year of the Scott catalog, it is a good idea to do so. Collections generally sell at low percentages of Scott value but they do sell at percentages, and this is a way to evaluate the offers that you receive. Further, it lets you contact dealers in an informed way and enables them to give some idea of their level of interest without you wasting a lot of your time. Modern US material should have the postage value counted.
Second in order of importance is to arrange your collection for sale. This means assembling it all in one place for shipment or evaluation if the buyer comes to your home. Put new issues in albums, if you have the supplements, or stock books if you don't. Glassine envelopes are the worst way to display your stamps. When you go to sell your home you paint it , mow the lawn, and pick up the trash off the floor. You would be amazed at the number of philatelic sellers who don't take the minimum steps to make their collections appear in the best way.
And lastly, most importantly, find a dealer who can properly handle your material. You need a dealer with wide ranging philatelic knowledge and established, time tested sales methods that will allow them to market your stamps in the best possible manor. If they can't get the best price for your stamps, they can't pay the best price for your stamps. They need to be financially sound, able to pay you immediately to any amount. And they need to be fair dealing, the best proof of this being, say, a hundred year record of satisfying stamp sellers. If it sounds like I'm describing Apfelbaum, you are right. We would be a very good place to start when you are thinking about selling your collection.