We all have our ways of trying to beat the system, but I've done this and had it work both ways: (got a last-day upgrade, and on other occasions an upgrade at the gate).

When you do a check in, and it asks you if you want to be upgraded, select yes. Then, exit without checking in. My experience has been that in 4 cases over the last few months:

2 - got upgraded the last night, got my regular "you've been upgraded" message
2 - got upgraded at the gate, either first or second on the list.

Maybe this process only seems to work for me, and really something else is going on. But I take no chances, since I always fly k class and the flights I take almost always have many, many elites (gold AND platinum) who don't make it. I do believe also that when you buy your ticket may have something to do with it.