I use Performance Trends - Roll Center Calculator. It's not their full blown Suspension analyzer software, but a lot less expensive and does 95% of what I need it to do.
http://www.performancetrends.com/
I sarted modeling mine on paper - scale drawings. That was an interesting exercise for a "first cut", but quickly became apparent that method was going to be too time consuming and tedious to give me the information I needed. The PT software really opened my eyes in seeing how small changes affect the geometry. You quickly realize that trying to "wing it" when laying out a suspension design could yield less than desirable results!
Jeff