Butt plug and cinch cord

This technique requires a bit of equipment and ingenuity, but it has the honor of giving me the best orgasm EVER.

First, you will need a butt plug with a flanged base.
If you are new to butt plugs, DON'T be afraid.  You do NOT want to waste your whole life away without ever discovering this thrill.  It does not make you gay.
As you use your plug, you will need larger and larger ones to keep getting the most out of them.  Be sure your bowels are reasonably empty and that you use lube on both the plug and your hole (Astroglide is good).  Work it in slowly.  Occasionally backing it out, twisting it, spreading your cheeks, or squatting can all ease the process...it also helps to be watching porn.  Getting over the widest part can hurt like hell, but once it's in you'll be kicked into a new level of arousal.  If it's the right size, you should be able to walk around without it dropping out.

Next, you will need a "cinch cord" like this one that I made from an old draw string and a cord lock.  I first designed this to be an adjustable cock ring, and it's great for that, but its partnership with the butt plug is incredible.
Open the loop of the cinch cord wide enough to encircle your entire cock and balls AND to fit around the flange of the butt plug already in your ass.  Be sure you are already hard so that your dick holds one end of the loop and the plug holds the other.  In this image, the blue thing is your cock ...if yours really is this blue, stop what you're doing, loosen the straps, and get some fresh air!

Now, cinch the cord down as tightly as you can.  This achieves 2 things.  It acts as a cock ring, restricting blood flow out of your cock, keeping it hard.  It also prevents the plug from being pushed out until you go soft (or loosen the cord).  The greatest pleasure of a butt plug comes when your body tries to push it out, but can't quite expel it past the widest part, and this will happen during orgasm!
To enhance things even more, yu can add a bit of auto-asphixiation.  Watch some porn, and when you estimate that the "best part" is about a minute away, take a deep breath and hold it.  When you feel you need more air, you can INHALE, but don't EXHALE yet.  The total fullness, combined with lack of oxygen can push you over the edge before you know it!  When you're done, let yourself exhale.

If you have trouble getting the plug out, relubricate your anus, reaching fingers inside if you can.  Try either squatting on the floor or sitting on a chair with your legs on a table.  I like to keep viewing porn because plug extraction feels damned good.  Push like you're taking a shit.  Pull the plug only if necessary.  I always thank the porn image who had the honor of helping me extract my plug.

You might get more than you bargained for...some brown stuff.  This is normal.  Just be prepared for cleanup.  Don't be caught doing this a minute before guests arrive!