The 300mm lens is one of the most venerable of the cycling shooters' lenses. In particular, Canon's 300mm f/2.8 IS is ultra sharp, has excellent bokeh, and is a dream to shoot with. You'll love your photos, sponsors will love your photos... but your wallet might balk at the $4000 price tag. Sites like lensrentals.com can get you one for special occasions.
#1 DON'T DO THIS!
Photo: Mitch Clinton, a much wiser and better photographer than me, who isn't stupid enough to do what I'm doing above
Yes, that's me doing something very stupid. I embarrass myself here so that you can know that much is learned through mistakes, mistakes we made last week.
Canon's 300mm f/2.8 weights 5.6lbs, which is far more weight than your camera mount is meant to support. Never let your camera body have the bear the weight of the lens. When you're carrying it around, always use the strap on the 300mm, not the strap on your camera body. If you try and carry it by both, it's likely to become separated.
Not all 300's are as heavy as Canon's f/2.8, but I thought I'd lead with this one as Mitch sent such a nice illustrative photo.
#2 Use Gaffer tape to cover the switches
This is a general tip which I'll repeat in other lessons
The 300mm has far more switches than you need. They are very easy to accidentally switch to the wrong position. Nothing is worse than going to take a shot and discovering your camera won't focus. By the time you figure out that the autofocus tripped to manual, the shot is gone. Get some gaffer tape and cover any switches you don't use all the time.
#3 Don't get too close
The 300's virtue is that it gets in real close to riders, but get too close and you'll soon discover that you have no depth of field to work with. It's also really, really hard to point a 300mm at short range. Make sure that you give yourself enough room that you can frame the rider from a distance.
#4 Think about scenics as well
A lot of people use the 300mm lens to get tight shots of riders. These make nice profile photos, but personally I love a good landscape that shows off the ride in addition to the rider. I actually get plenty of bang for the buck using the 300mm as a scenic lens.
Is there a mountain in the distance? Try and see if your 300mm can frame it with a rider in it. Stuck in a pretty field? See if your 300mm can collapse it into your frame:
#5 The 300 is great from a car
You may feel stupid shooting out a car window with a 300, but when you have to get a shot of a rider that's 10 cars away you'll love it.
#6 Monopods are for wimps
Actually, I'm a wimp, but I still won't use a monopod as cycling is about catching the moment... which you're likely to miss if you're busy futzing with your monopod. Sometimes the action will be down low, up high, around a corner, who knows -- but you don't want a monopod getting in the way while you're running around figure out where it is.