bartjoosen said:...So how can I train for the start, ...
The start can be really important depending on the cyclocross course, the amount of single track, the passing opportunities or lack thereof. Winning the hole shot on a technical course or at least entering the tight sections in the first few riders can make all the difference if things stack up or slower riders hold you back.
If you want to improve your starts, practice starts. Find a grassy field with up to a couple of hundred meters of straight away before a hard turn or narrow section. Start with one foot on the ground and practice hammering off the line, pedaling hard whether or not you immediately engage your pedal and giving everything for that initial acceleration. Try a few of these as hard as you can for the start section, but take it a step further and do some of these simulated starts with another two to three minutes of race pace riding on a mocked up cross course using natural features like trees to mark turns. IOW, go really hard at the start but also learn to settle right into race pace.
Cross is like a race turned upside down, you've got to sprint the start on tight courses but you've also got to settle into a full Threshold or higher race pace effort after that 'sprint' which is really tough but it's the way the game is played. It sure helps to get a callup for a start near the front but as you've noticed you've got to earn it at many races. You should also preride all courses and look for passing opportunities so you know exactly when to punch it to get around folks. Some courses have a lot more of these than others and you can use these to move up in the rankings to improve your future start positions so pick your courses if you have to start at the back.
Sometimes they just skip the callups like my race this morning where the officials were running a bit behind and just started us where we were. Definitely a bummer for those of us with series points but good for folks who started later in the season so always line up near the front unless they specifically tell you they'll be calling riders up for start order. You never know...
Good luck,
-Dave