I'd remark to the OP that it gets better once you hit PM, get access to C+ at booking and theoretically are higher on the list for upgrades. The thing you have to realize is for markets like LGA-MSP, BOS-ATL (and indeed it appears BOS-SLC) during peak times even a good number of Diamonds are sitting in back. During off-peak times and for personal trips I've experienced a very high upgrade success rate.

I travel weekly and rarely book more than a week out myself - if you keep an eye on the seat map you can usually get something decent in C+.