I used to be so impatient about getting published. I've already submitted my first novel to multiple agents, and been rejected multiple times (though some of them were very nice rejections, if I do say so myself). But waiting (an important skill in the publishing business) is not wasted. Here's why.
First of all, time gives me distance from my manuscripts. I can be more objective about their faults when there is some time between me and my 200-page babies. I can also be more merciless when I need to tear it apart in order to fix it.
Secondly, I can work on other projects, such as new manuscripts or articles. This gives me added credibility if these projects get published (yo, someone else paid me money, so you can, too). This also gives me something else to work on so it doesn't hurt so much when the U.S. Post Office drives through sleet and hail to bring those SASEs back to me. Another advantage is that when I do finally get that multi-book, multi-million dollar deal, I already have several ideas developing so I don't have to pull more manuscripts out of my butt.
Last, I get to know other writing types. I used to scoff at writing books that advised joining writing groups, but I have discovered they are right. Going to conferences, emailing other authors, and attending other writing events are also good ideas. Connections are a good thing. They can get you in the door, help fix your manuscripts, and give you really good practical advice.
After determination, I vote for patience as the #2 most essential virtue for writers.