There's no way around it. You have to spend money on your sound. Hire the best sound mixer that you can afford, and maybe squeeze out a little more money to get an even better one. I have found that it's really hard to get a decent sound mixer for less than $250/day. I will pay for sound because bad sound will make your film unwatchable and then what's the point?
As for post production sound, it is a significant challenge to find quality post production sound for small indie films. It usually takes some intense negotiating, begging and pleading. Even then, you are talking about spending at least $15 to $25 grand on the very low end. A solid post production mix is really closer to $40 grand on the low end. I know that is a lot of money. It hurts to write that check. But sound is incredibly important to the success of your film.
If your production sound is good quality that will really help to keep the post sound work less expensive. And many times, filmmakers can't afford a post sound mix so they have to rely on their production sound. So be sure to at least hire a strong production sound mixer so you can screen at festivals without having to do a post sound mix.
Also, try as hard as you can to secure quiet locations. Make sure there are no train tracks nearby, a loud freeway, or an airport. There's nothing like ruining take after take of the sound due to filming in the flight path of an airport. Filming near water is killer on your sound as well. What sounds like a beautiful rush of water can have your actors yelling to be heard and suddenly a tranquil scene turns into a shouting match. And if you film in the South, resign yourself to the possibility of hearing cicadas throughout! Oh, how I love cicadas.