to begin with you need a reliable bike. I have a 9 year bike, bought with my London Cycling Campaign discount from Evans Cycles in The Cut near The London Eye at Waterloo.
Here is a picture of my cycle, loaded up with: two waterproof Ortlieb pannier bags, a couple of litres of water, and a stuff-it pack with quick release water-proof for those times when it pours. I also made sure that I had new puncture resistant tyres and new brake pads, a couple of spare inner tubes and a repair kit. I gambled by not taking along any spare spokes, but these are relatively easy to come by in France.