AO27 has a very sensitive receiver, and I have managed to hit it with 100 mw of power at times. Since AO27 is a very busy satellite, especially on holidays and weekends, a portable operator will most likely have limited success, unless they use a multi-element gain antenna for the uplink and downlink. I have also found use of a preamplifier beneficial for hearing the bird well, and knowing when to jump in for a contact. Most QSOs are short, contest style exchanges, including grid square, first name, and city. This sort of contact maximizes use of the bird for the scores of operators who might be trying.
Weekend passes, to the uninitiated, sound a bit chaotic.
There is a culture, which exists on the bird, and my best advice is to listen to a couple passes, and model your operation after the successful operators you hear. AO27 is a full duplex bird, meaning that you can listen to the downlink as you transmit. If your rig is capable of full duplex operation, fine, otherwise half-duplex operation works just fine on the FM birds if you program the frequencies in your HT correctly. You should program your HT with 5 frequency pairs for working AO-27. Start with the first pair, using a non-standard split, and tune to the next one as the pass progresses.