The information contained here is courtesy of www.gnttype.org -- it's really a kick ass website for car fans :)

Good explanation of "Why use toluene?"

How to calculate final Octane:

    (Initial gallons * gasoline octane) + (Added gallons * additive octane)
    ------------------------------------------------------------------------  =  Final Octane
                        Initial gallons + Added gallons

Fast facts: Automotive gasoline is ~6.2 lbs/gal, Aviation gasoline is ~5.9 lbs/gal

Don't use it unless you have no other choice -- because the PCM doesn't know the density is lower, you'll run lean, maybe TOO lean

  USE AT YOUR OWN RISK!!!!

Richard Lassiter's "How to Mix Your Own Brew" 

FORMULA 1
Toulene
R+M/2...114
Cost...$2.50/gal
Mixtures with 92 Octane Premium
10%...94.2 Octane
20%...96.4 Octane
30%...98.6 Octane
Notes: Common ingredient in Octane Boosters in a can. 
       12-16 ounces will only raise octane 2-3 *points*, 
       I.e. from 92 to 92.3. Often costs $3-5 for 12-16 ounces, 
       when it can be purchased for less than $3/gal at 
       chemical supply houses or paint stores. 

FORMULA 2
Xylene
R+M/2...117
Cost...$2.75/gal
Mixtures with 92 Octane Premium
10%...94.5 Octane
20%...97.0 Octane
30%...99.5 Octane
Notes: Similar to Toulene. 12-16 ounces will only raise 
       octane 2-3 *points*, I.e. from 92 to 92.3. Usually mixed 
       with Toulene and advertised as *race formula*. 

FORMULA 3
Methyl-tertiary-butyl-ether (MTBE)
R+M/2...118
Cost...$3.50/gal
Mixtures with 92 Octane Premium
10%...94.6 Octane
20%...97.2 Octane
30%...99.8 Octane
Notes: Oxygenate. Very common in octane booster products. 
       Has lower BTU content than toulene or xylene, but oxygenate 
       effect makes the gasoline burn better and produce more energy. 

FORMULA 4
Methanol or Ethanol
R+M/2...101
Cost...$0.60 - $1.75/gal
Mixtures with 92 Octane Premium
10%...94.3 Octane (Methanol)
10%...94.7 Octane (Ethanol)
20%...Not Recommended
Notes: Methanol is wood alcohol. Ethanol is grain alcohol and found 
       in Gasohol in 10% ratios. Both alcohols are mildly corrosive 
       and will eat gas tank linings, rubber and aluminum if used in 
       excessive ratios. Main ingredient in "Gas Dryers", combines 
       with water. 

FORMULA 5
Isopropyl Alcohol and Tertiary Butyl Alcohol
R+M/2...101
Cost...$0.60-$1.50/gal
Mixtures with 92 Octane Premium
10%...94.5 Octane
20%...Not Recommended
30%...Not Recommended
Notes: Similar to Methanol/Ethanol. Isopropyl Alcohol is simply 
       rubbing alcohol. 
 
  
MAKE YOUR OWN OCTANE BOOST
How to make your own octane booster (this is the basic formula of 
one of the popular octane booster products). To make eight 16 ounce 
bottles (128 oz = 1 gal): 
     100 oz of toulene for octane boost
     25 oz of mineral spirits (cleaning agent)
     3 oz of transmission fluid (lubricating agent)

This product is advertised as "octane booster with cleaning agent 
*and* lubricating agent!". Diesel fuel or kerosene can be substituted 
for mineral spirits and light turbine oil can be substituted for 
transmission fluid. Color can be added with petroleum dyes.