1) Number your measures: if your score does not already list measure numbers, write down the measure number corresponding to the first measure of each line before submitting the score to the pianist. This will save you a lot of time during rehearsals.
2) Photocopy your music/send originals as opposed to sending PDF copies via email whenever possible unless otherwise requested by the pianist.
The reason is that this saves the time and paper-ink resources of the pianist. While it is not a big deal when dealing with one or two pieces, if they have multiple scores to print out, it can take a lot of time and resources for the pianist. The pianists I worked with in the past have all been happier (and more willing to work with you) when I showed them this courtesy. I highly recommend it where applicable.
3) Double sided copies are better than single sided copies for most pianists. Make sure there is enough space on the margins in order to insure that the music is legible, if the pianist decides to hole-punch the music.
4) Record your rehearsals. You can also ask them to play just the accompaniment so you can play along to the piano part at home. Please ask the pianist first, before turning on the recorder--it is most courteous and professional to ask first.
5) Be on time, with your flutes ready to go.
6) Bring a pencil.
7) Always thank them for their collaboration after the rehearsal. Shake hands, where applicable. The pianist is your best friend on the stage! Manners are important.