I've had this mod on my cars for over 15 years.  I've changed houses three times and cars at least that many, and I still do it to every car (house) I have.  This was done to my S on day 3.

What it does:

This mod connects your garage door opener to the high beams, so you can open the garage door by 'flashing' the high beams.  Very Cool.  And no more batteries to buy.
Note - sorry this mod does not work on cars with DRL (daytime running lights).

Option 1 - use one of your Cruise buttons instead of the high beam switch:

The Cruise buttons aren't as handy to operate as the high beam switch, but if you don't want to be "flashing" your neighbors you may want to try this option.  Or as an option for owners who use their high beams a lot and are concerned about continuously operating the garage remote.  This option is wired under the driver's dash instead of under the hood and gets +12v from the Cruise button.  See instructions here.

Skills and tools required to install:

Parts required for 12v or 9v remotes:

All part numbers are for Radio Shack.

Additional parts required for 6v or 3v remotes:

See instructions here on how to wire this up, it's not too hard, only a few parts.

Basic Installation Procedures :

First, you'll need to decide where you want to mount your remote.  You can access the high beam wires from either fender well, the left inside fender by the master cylinder, or inside the car under the dash.  I usually put mine in the engine compartment to get better range, but this varies with different vehicles.  I figured the rf signal from the remote has to penetrate more stuff if it's inside the car, but that may not be your case.

Check out the photos below and see how you want to tap into the high beams.  Bear in mind you will have to connect both positive and negative wires to the high beam circuit.  You can't run the remote negative power wire to ground, because the high beams have +12 volts if the low beams are on.  Honda decided to complicate the wiring by switching positive with the low beam switch and negative with the high beam switch.  Go figure.

1. Test your remote, with the battery installed, in the location you will use to mount it.  You need to make sure it will transmit through whatever metal is between it and your garage door opener.  Try it at the end of your drive, and if you will be mounting it under the hood, do it with the hood closed.  You can hold the remote up in the engine bay from under the car.  If it works OK, proceed.  If not, too bad.
2. Take apart your remote, remove the battery, and solder a jumper wire across the switch contacts.  I can't show you a photo of this because there are so many different remote styles.
3. If you have a 9-volt style remote, attach the battery clip to the remote's battery clip (it's easier than soldering wires).  Pay attention here, the red wire on the "extra" battery clip is negative, the black wire is positive.
4. If your remote is a 12-volt style, you'll need to solder wires to the battery connections on the remote.
5. Connect the battery wires from the remote to the high beam wires.  There are several places you can do this:

     Inside left fender, by master cylinder.  See photos A & B.  You want the red/yellow (positive) and red/blue (negative) wires.
   
     Inside the car, under the dash.  See photos C & D.  Again, you want the red/yellow (positive) and red/blue (negative) wires, on connector C301.  This location is more difficult to connect the wires, but gives you 'inside' installation of the remote if you find a place for it under the dash.
   
     Inside either fender, behind the headlights.  This is where I put mine.  See photos E & F.  You want the red/yellow (positive) and red/blue (negative) wires on the left (driver) side, or the red/white (positive) and red/blue (negative) wires on the right (passenger) side.  Look for the wires in the wire bundle coming from the high beam (duh).  Make sure you get the polarity right, you fry fewer remotes that way.

6. Put the remote in a plastic bag and mount it somewhere.  If you look at photo F you can see I just tie-wrapped mine in the fender.  First Class, huh?

You may click a photo for an enlargement.

Option 2 - installing a 'safety' diode for the timid:

I have used 12-volt style remotes and 9-volt style, both powered from the car's 12 volt system, and have never had a problem.  It's possible that some 9-volt style remotes will burn out prematurely if you power them from the car's 12 volt system, I just don't know.  And I don't care, they're your remotes, not mine.  (just kidding).  I would be surprised if they burned out, but it's possible.  

If your remote is the 9-volt style and you want to be absolutely sure it will survive this mod, you can purchase the 5.1 volt zener diode (in the parts list) from Radio Shack, and install it in series with the positive power wire going to the remote.  See photo G. The zener will drop 5 volts, meaning if you supply 12 volts, the remote sees 7 volts.  In most cases the car's electrical system will be supplying 13.5 to 14 volts, so the remote will operate on 8.5 to 9 volts, right in the 'normal' voltage range for a 9-volt device.

You cannot use a regular diode, you must use the zener diode specified.  A regular diode will not work at all if you wire it like I have shown.  If you wire it backwards, it will work, but it will only drop 0.7 volts instead of the 5 volts like the zener does, so there's no benefit.