It's a bit more complicated than the above, so look at guides on using GPU BOOST 2.
There's a POWER TARGET which may limit the overclock, so you may need to raise that too. As said below though, that may not work so basically the above would then be correct (raise GPU and Memory).
I wouldn't drop it only 10MHz below where it crashes though. I would suggest starting with something like Unigine Valley Benchmark set to run in a loop. Just run until the temperature stabilizes on each test.
I'd go no higher than 50MHz below crashing but maybe that's just me. For example, if it crashed at 1300MHz choose 1250MHz. Don't forget the memory and GPU should be roughly PROPORTIONAL so go by percent for both (5% on each maybe to start).
So again, maybe go up 5% or so until crashing, then go back to the last stable and go up 2% or so because otherwise you might actually be right on the verge of crashing. Now that you have a better idea of the crash point back off 5% or so.
http://www.hardocp.com/article/2014/09/15/asus_strix_gtx_750_ti_oc_edition_video_card_review/3#.Vy6qTZUrL1s