As Rainmaker and Seineseeker say - get there as early as possible.
I've done it a couple of times and the first time it was just me and my wife, we were at the end of the Champs Elysee where they turn 180 degrees. Great spot right at the front but we got there before 1100 and after 2 pm there was no way to get out and then back in even with the other person saving your space. Definitely a good place to watch from, the final vehicle was a Gendarme van which everyone cheered and encouraged the driver to go faster each time he went round, by the end he had it on two wheels!
The second time four of us went and were able to secure a good spot, again well before lunch and being a bigger group it was easier to get out for food, drink and de-watering.