This was a bit tricky. At first i tried copper wire, push pins and various other things. With these experiments i determined that you need to expose a small and consistant amount of the probes to the solution at a consistant distance. I accomplished this by using alot of hot glue and/or insulation on the wire so that only the tips of the wires would touch the water. Then I realized that the stupid readings drift like crazy untill you clean the probe again, and even cleaning the probe sometimes didnt help because probably some chemical reaction with the stuff in the solutions. I kept trying to think of something that would be a great conductor without corroding in the solution, and came up with GOLD! I used a few types of gold plated audio connectors before i found some bananna jacks that seemed to work best. I covered them so again only the tips are exposed to the solution at about 1cm apart and was able to get consistant readings over and over especially if i cleaned the tip before and after each test.
So, use gold plated probes, make sure a small and consistant amount of probe touches the solution, and clean the probes after and before each use. Using these guidelines should get you a nice reliable probe, I had wanted to find a nice premade and ez to find connector that i could suggest, but nothing laying around here is very common. Gold plated RCA plugs seem to be the worst probably because these cheep things are only plated enough to look gold colored. Was looking for a nice gold plated 1/8" phone jack or something but nothing was laying about here blah. If the meter cannot seem to be adjusted right (is allways too high a reading) try increasing the spacing of the probe or changing values of R7. If reading too low try moving probes closer or exposing more probe material.