I always look for the biggest 1st Class section and make sure I choose the flight that would offer the best chances. If it looked like it would be difficult to be upgraded, I would use those dusty miles sitting in my account. But honestly, I have had a really good upgrade success on that flight. I also love it b/c it's almost always a "cookie flight" for me.

I prefer flying DTW-EWR-LAS than the IAH route. But I have to admit, I am headed there Tuesday via IAH and I got my upgrades for both segments 5 days before. I do think that the CLE-LAS was always easier for Plats (IMO) but I haven't seen that route offered in a while.

Truth is, EWR-LAS really is an iffy upgrade. But I have had times where I've upgraded with miles and then been furious with myself because there have been empty seats.

Good Luck- hope you get the cookie flight!