One more year left before I apply to math graduate school. Any advice to obtain good recommendation letters? I am in the last year of my undergraduate studies before I apply to graduate school. I have some questions and I am also in need of some advice. 

I know one professor very well and I plan on asking him for a recommendation letter. I also plan on doing research with him next summer. But I need two more and I am not quite close with any other faculty members. 


Are letters from professors weighted more heavily than letters from postdocs? 
Is one class enough to get to know a professor/instructor well? Would two classes be enough? 
What if I don't take a class with a professor but I do a reading course? Is a single term of a reading course good enough for a professor to know me?
I'm a mathematician who has been on a graduate admissions committee (albeit just for a masters program) and have written recommendation letters also. To answer your questions:


Yes, letters from professors may be weighted more heavily than postdocs, but the difference will be tiny; this won't likely matter to everyone looking at things. The biggest differences will likely be twofold: first, people on the relevant committees are more likely to actually know a professor than a postdoc, and if they do, that will generally be a positive. Second, if a professor says "This is the best student I've ever had" or something similar, that will mean more from a professor than a postdoc who hasn't taught as much. That said, a good letter from a postdoc will matter more than a mediocre letter from a professor. 
One class is enough, two or more is better. If you only have had one class, make sure to stop by their office hours, ask good questions, and generally excel in their class. 
Yes, and if anything, a single reading course may be better than a single regular course since they'll have more one-on-one interaction.