Welcome to Flyertalk jdl196. First thing is to decide do you want extra miles or extra segments -- sometimes you don't get both (eg. ORD-DEN -SFO is two segments -- assuming both flight numbers are different -- but the same miles as ORD-SFO nonstop.) 1wfudeac's suggestions are great but I believe UA routing rules will not allow his trip routing -- therefore from MSN I'd say "go West young man". MSN-ORD-SEA-PDX-SFO ( as an exanple) will give you segments and miles.
The trick is, of course, finding all the segments with available fare buckets to keep the cost under control. I've also found that on occasion using the UA web site (.bomb as its often called by FT's) and putting in city-pairs with "2-stop" default will sometimes give you surprisingly creative routings and decent fares. Yes, its a lot of work -- but thats part of the game we all play.
Good luck.