Let go if you can't do anything about the situation.
If there isn't anything that you can do to resolve whatever has triggered your impatience, just let it go. Easier said than done, yes, but it's possible, and it's the only healthy thing to do.
Initially, you will probably find it difficult to let go if the matter is important to you — waiting to hear back after a job interview, for instance — but you should be able to alleviate impatience that's caused by issues of less consequence (i.e. waiting in line at the grocery store).
If you make a concerted effort to be more patient in relatively inconsequential, short-term situations, you'll gradually develop the strength to remain patient in even the most trying and enduring situations.