In reply to Dr Avinash Aujayeb:
Was writing on my mobile earlier, so here's a bit more info.
There's plenty of publicity and souvenir stalls around in the 2-3 days before and they start building all the stands in advance. The day before is when they start closing some bits of road to traffic in order to build the bigger stands, such as Place de la Concorde. On the morning of the event, if you have a bike and are early enough, you can cycle round the course on pretty much closed roads until they really start closing sections to finish the setting up. You'll be doing it with lots of other people and it's a really good atmosphere.
You'll need to get there pretty early to bag a good spot. Try and pick a shady-ish spot where you can see one of the big screens, so you can see the racing and everything else. Take plenty of water/food and sunscreen/waterproofs accordingly.
From experience, try not to be too close to big groups of Dutch supporters - they're very nice, but drunkenly shouted choruses of the same rude song about the national champion gets a bit wearing after the 100th time.
I was on the finish straight in Pau in 2012 and we saw interviews with Chris Boardman, a few junior races, loads of sponsors chucking out keyrings, t-shirts, hats and so on. If you do well, you can get fed (sausage, cakes, sweets, ice creams) and watered (Vittel and others) free!
Not sure how much truth there is in it, but I've been told that in Paris, as it's the end of the Tour, lots of the teams literally bin stuff they don't need anymore, so you could get lucky with all sorts of swag.