Professionals (like me) will always do a better job, but as the first post points out, window cleaning is a lucrative business so professional work is not done cheap.
A couple tablespoons of dawn dish soap mixed with a few gallons of water is what the vast majority of pros use. We also use Ettore or Sorbo squeegees with pretty much any applicator, but I prefer Unger. These, with lots of clean dry rags, and you'll be good to go.
However, if you have little or no experience with washing windows, I recommend sticking to straight-pulls from left to right (if your right handed,otherwise vice-versa), and wiping your squeegee with a rag between each pull. The fanning technique is much more efficient but it does take about 40+ hrs to get the hang of, and many months to master.