Practice, practice, practice. I'm a geek about software and own all kinds of it both desktop and portable, but at the end of the day you don't NEED to buy software because there are plenty of good freebee web based calculators out there. It's just a nice to have. No matter what, you are always better off building your own drop table from empirical data.
A good practice tool for mil dot ranging is available at Long Range Shooting Simulation Demo
A few words on why this tool is limited in it's usefulness: you do not have to worry about steadying the rifle and the target sizes are known exactly. Even with that, you will find you can still get it wrong. These are the things that make a mil-dot or any range finding reticle only a good estimate. Once you get the concept down pat with the software, it becomes second nature as far as making a range calc and then you are ready to start using your rifle to practice with. One of the things I hardly ever see mentioned with these is up and down hill shooting and how it effectively makes your target smaller and how this angular difference plays into using a mildot for range estimation. Most don't shoot more than 5 degrees up or down anyway so for many it is not an issue, just something to be aware of.
If you really plan to shoot or hunt in field conditions at unknown distances, I'd recommend a laser range finder, these have some serious user limitations too though if you don't understand how they work and what the beam size is at a given range.