Just don't look down for too long, or they'll probably fall off your face. I regret getting my current oakley eyeglasses, the lack of wrapping around the back of the ear is a real obnoxious trademark design flaw.
MooTatersCouple things:
You may have chosen the wrong sized frame. In your case, probably too large.
You may have a very heavy pair of lenses due to the strength of your prescription. A high index lenses may help.
I have multiple pairs of glasses including Oakleys with straight temples and they do not fall off while looking down unless I shake my head or am sweaty.
JijaYaninMine rarely fall off, but that's because I push them back up(which gets annoying) before they get the chance, though they have fallen off from sweat and a jerk of the head a few times...while looking down of course. My lenses are high index(practically required for my Rx), but I was also told that would actually make them heavier, which from a quick google search seems to be true. For reference, my Oakleys are the TinCup frames.
JijaYaninI'll be honest and add, maybe a week after this give or take the weld near the upper right corner of the lens did come loose. It had seemed that possibly that happened over time, but at very least after repair they do sit better on my face. That said I still rarely feel they're as securely held in place as other glasses I've owned in the past, and that has always led to me trying to be cautious of them falling off, which has still happened anyways.