Good advice already given, but here are a few more suggestions:

1) Adjust the rearsight (roughly) for the distance before you start. Shooting at 100 yards with a rifle zeroed at 25 yards, the shots will hit about 8in low of centre. This is enough to miss the sighter completely and to clip the top of the upper scoring target. It usually takes several shots for the poor unfortunate who's done it to realise.

You need to turn the rearsight about 8 minutes UP from your 25 yard zero; this equates to about 50 clicks anti-clockwise with an older Anschutz, or 100 clicks anti-clockwise with a newer anschutz.

2) Take your time over your sighters. The sights won't be properly zeroed for distance or elevation, so allow more shots than you would normally (at least 10, plus a few extra). You need to know where your group is, and how large it is (it will be much larger than at 25 yards - probably 1in or larger). Fire three or four shots (all carefully of course), and then count the rings between the group centre and the target centre. For an older Anschutz rearsight, turn the turrets 4 clicks per ring (vertical and horizontal). Don't apply one click at a time, this is just too small and will be lost within the spread of the group, and your aiming error.

This should see you reasonably well centred (fingers crossed). Fire another couple of sighters, and make further adjustments as needed.

3) If your spotting scope isn't very good, try to borrow a posher one from somebody. If the weather is warm and very sunny, it can be hard to see your shots on the target. Quality glass helps here.

4) Make sure that your position is good, and that the rifle points naturally at the target, BEFORE you load your first shot. Getting your position aligned with the target will make it less tiring and easier to shoot well. It's also not a good idea to try a new rifle, sling, or jacket on the day!

5) Take lots of water with you. It may be hot this weekend, and dehydration (even slightly) will degrade your vision.

p.s. Good luck, and enjoy yourself.