Put a little money aside every month - better yet, set up an automatic transfer so it's seamless.
Tax returns or bonuses can make a good nest egg for an engagement ring purchase - so keep on living like you didn't get the influx of spending money if you can.
Cut out unnecessary spending. Is there something frivolous you're doing every day that you could put on hold in order to save a little more cash for that dream ring?
Divide the ring cost by number of months you have until the proposal -- this tells you how much you'll need to save per month.
Set up a new account specifically for your ring goal. Don't mix your spending money with funds earmarked for the ring. Out of sight, out of mind (and ability to accidentally spend!)
If you're thinking a year or more ahead and/or need a lot saved up -- explore higher interest savings options, like high yield savings accounts that allow easy withdrawals but give a larger than normal return.
Keep control of your emotions. Engagement rings are sentimental objects and we know you're excited -- but reign it in and keep your budget and savings top of mind while you're shopping.