All great suggestions, including "bring a chair if you need to". Some of the greats play sitting down (BB King and Robert Fripp come to mind).

Sam makes a good point about finding a good position by adjusting the strap. Everybody wants to look cool like Jimmy Page and have the guitar slung down low, but it's really not practical, especially for a beginner.

Many great players have their guitar higher. Look at John Petrucci from Dream theater - he's a virtuoso player and has his guitar strapped pretty high up on his body. He is far from dorky looking when he plays :-). Early pictures of John Lennon show his guitar (both electric and acoustic) positioned practically to his chin.

When I started with guitar tricks, I experimented with adjustring my guitars to be higher up and it made a big difference. In the core lessons, Chris Schlegel has a good tutorial on adjusting your guitar strap and fine tuning your playing position. You might want to check it out.

The routine I've fallen into is I will sit on a stool when going through lessons. When I'm practicing what I've learned or jamming to a background track, I'll stand. It's worked for me.

Good luck!