IMO, There is no reason to spend a $100 or more for a reverse lock out relay. A lot of people wire the lock out to the brakes but the one thing that I do not like about that approach is that the solenoid is activated every single time you press the brake pedal regardless of whether or not you are actually shifting to reverse. I took a different approach, I used a shift knob equipped with a line lock button and wired that to the reverse lock out solenoid. This shift knob button works exceptionally well since your hand is already on the knob when shifting so it is a natural movement to press and hold the button with your thumb when shifting to reverse. Of course this approach assumes you are not already using such a knob for a line lock or nitrous system.
I'm using this Speeddawg knob but you can buy line lock knobs anywhere.
http://www.speeddawg.com/pro-white-b...ck-nos-button/