Update December 2012: I added a video at the bottom. Check it out to see the assembly and disassembly of the front take down pin.
Installing the front takedown pin on an AR-15 can be one of the most frustrating parts of assembling the lower receiver. Before you can install the pin, you have to compress a spring and detent. If you accidently let the detent slip, it will probably shoot across the room and be lost forever. There are several different tools you can buy from some of the bigger gun part companies, such as Brownell's. While these tools do exactly what they need to do, I feel like they are surprisingly expensive.
While doing some research online for my first AR-15 build a couple years ago, I came across the suggestion to use a clevis pin. Essentially is a 1/4" diameter pin with multiple holes in it. You should be able to find it at your local Home Depot or Lowe's for around $1.
Below is an illustrated guide showing you how easy the pin is to use:
The first step is to insert the pin and place the takedown spring through one of the holes. The spring will slide most of the way in and there will be very little to none of the spring sticking past the pin.
With your spring in place, next place the detent in front of the spring and use a punch to press the detent back into the receiver. The punch doesn't have to be the same size as the detent, but make sure that it fits inside the hole in the pin. I like to use a punch that is about the same diameter as the hole so I don't have to worry about the detent jamming between the punch and the wall of the pin.
Once the detent is clear of the pin, use the punch to rotate the pin 90 degrees. The detent should now be pressing against the smooth side of the pin. The nice thing about doing it this way is you don't have to hold it in place, you can easily keep your hands free to ready the takedown pin.
Now use the takedown pin to push your place holding pin out of the way. This is the only time of the install that you could lose control of the detent and spring. I'm only using one hand in the picture for clarity, but normally I use my other hand to brace the gap between the two pins so that the detent can't slip by.
Once the takedown pin is inside the ear of the lower receiver, you are home free. Now just make sure that the takedown pin is rotated so that the detent rests in the built in groove. Now your takedown pin is ready for the upper receiver.
If you haven't used this method before I hope that it will make your next lower receiver assembly a little easier. I may add some additional lower receiver guides in the future, but there is a lot of info out there already.
Let me know if there are any parts of the install that you would like to see documented in photo or video form.
The lower receiver in the pictures is painted with Brownell's Aluma Hyde II in Coyote. I will post some additional photos of the assembled gun later, but you can check out a previous post where I compared it to Magpul Flat Dark Earth (FDE) accessories here.