Let's look at this in recipe form...
The first important step in creation of a big bluegill is to start with a fish that has the right genetics. By genetics, I mean that the fish should come from a strain that's well adapted to your particular region and climate. Don't use coppernose bluegill in Minnesota; Dont import bluegill from Arkansas for your Oregon pond. Get healthy fish from a reputable source, and if you're stocking a pond that doesn't currently have bluegill (or any other fish yet) try to get fish that are at the top of their class. Offer to pay your supplier a little extra for fish that are sorted from the top of their particular class. If your supplier runs a seine through a pond full of late age-0's, ask if you can pay an extra nickel or dime per head to get the largest ones. These fish will often be more agressive, dominant fish and in my experience will be a little heavy in males.
The second important step is to do whatever is necessary to assure good quality water. Your water should be clean, free of heavy run-off sources, possibly aerated, and with Secchi readings in the 12-24 inch range. Clear enough for sight predators like bluegill to thrive, but also fertile enough to support high volume production of invertebrates and other preferred prey items. It cannot be overemphasized how important it is to have consistently oxygenated water. If a bluegill is going to take 5 years to reach trophy size, all it takes is one hour every five years of no oxygen to ruin your project.
The next step is to develop an appreciation for how much food your fish are going to require. You must assure that your bluegill do not overpopulate, and the best way to do this is to keep heavy pressure on .5 to 4 inch bluegill with the introduction of an appropriate predator. In many ponds the ideal predator is a "lean and mean" population of largemouth bass. High density populations of 10-14 inch largemouth bass (LMB) will hammer on your YOY bluegills to the point that many fewer individuals survive to compete for the available resources (food items). Once bluegill reach 5 inches, you as an angler can start to have an influence and I highly recommend a program to place maximum pressure on adult female bluegill. Learn to ID fish by sex, and harvest of females has a two-tiered benefit. It will reduce overall number of individual bluegill to compete with one another, but it also leaves numerous dominant males to take up the available prime spawning grounds. This will force immature males to continue expending energy towards growth instead of nest building, nest guarding and gamete formation. It is extremely important that bluegill never go long periods of time without preferred food items. You may, in some circumstances use a pelleted artificial feed such as Aquamax grower 500 or 600 for bigger adults to supplement the diet of your adult fish, especially during periodic lags in invertebrate production. I strongly believe that the biggest bluegill will utilize BOTH artificial and natural feed. Bluegill that go months without good nutrition can recover later on, but will likely have a lower top-end potential.
The fourth step is to protect your best fish. You should probably consider using barbless hooks when possible, and limit the overall amount of fishing that you do in the water body in which you wish to grow the monster bluegill. Fish that are heavily pressured can be more susceptible to disease and injury, and also ultimatley become harder to catch.
Finally learn to identify your fish by general body condition, or relative weight. If you really want to be skilled in your harvest efforts you will learn to harvest fish with lower relative weights and release the fish with higher relative weights that are thriving in your given pond or small water body.
Follow these rules to give yourself the very best chance to grow the bluegill of a lifetime.