I'm memorizing the Haydn Trumpet Concerto for a performance in June. I'm rehearsing with the orchestral backing on an MP3 player, actually performing with a pianist on the day. Once I actually start playing I'm OK - but I'm still not 100% certain where every entry comes in. What's the best way to come in at the right moment? Memorize the number of bars rest to count between entries? Memorize the accompaniment? Or some other technique?
I both memorize the accompaniment and know my "cues" and count. Ideally one would have the whole piece in one's head and just know/feel when to come in, but with the typical amount of rehearsal time available, it's often wise to count to be certain.
You can also use a hybrid system. If you know a cue (an easy to recognize moment played by someone else) and exactly where it is in the timing, then you can count from the cue to make sure you come in at the exact right time without having to count out 45 measures or whatever.
The pianist will have the score with both parts showing. He/she will be able to nod you in. It won't look bad, as it'll be rather like 'I've done my part, now I'm handing back to you'. You could actually reciprocate, making it look like proper teamwork. Other than that, the existing answers seem to cover most other options.
What I have found works is to do Harmonic Analysis of the score that you want to memorise. There is great parallels as to how actors learn long plays.
When actors for example want to memorise a Shakespeare play they visualise each scene in their minds eye. The mind has great potential for memory when the visual approach is used.
You can learn to visualise score in the same way actors learn to visualise plays. I remember seeing a famous pianist talking about how she was taught from a young age to visualise scores and this was a great help in learning long pieces quickly.
It got so good for her that she could remember the exact font the letters were written in and all the little imperfections of the paper the music was printed on. You truly can have an Eidetic musical memory
So to bring it back to music. You have to go further than seeing the scores just as a collection of notes. You have to see the devices that the composers use to communicate the feelings and the message of the music.
So for this to happen I would start by figuring the score or in other words working out the chords. You may have to look at the complete score, not just your part. Also take note of the inversions
See how the melody works. Look at what voices takes the melody and which provide the harmony.Take note of all the melodic devices that are used to make the melodies interesting.
Analyses the score.
When you do this you stop having to force the memory of these notes that you only somewhat comprehend but rather learn true insight into the music.
Insight is what it all comes down to.