I have exactly same problem and have tried a cock ring several times with mixed results.
First of all, I discovered that for me there is no effect at all unless I put it around my balls as well as my cock. Research online shows that this is an accepted method, but that makes it hellishly tight and you don't want to have it in place there for longer than 10-15 minutes.
More recently, I've read that it works better if you put it on before you get fully hard. That seems counter-intuitive to me – isn't it there to trap the blood that's already flowed in? – but I will try it out at some point.
On a related subject, I've also found that drinking any more than the driving limit for alcohol drastically reduces my erection capability, even with a blue pill. So bear that in mind too if you ever have a pre-punt drink.