To increase the probability of a QSO if copy is difficult, I find that the
following procedure works well:
1) start and end the first transmission(s) with 3 x 3 callsigns (e.g. N4ICK
N4ICK N4ICK DE GW4ALG GW4ALG GW4ALG);
2) once you are sure that both callsigns have been copied correctly, start
and end each over with 1 x 1 callsigns (e.g. ZL4MD DE GW4ALG);
3) if you think that one of the callsigns is in doubt, use 1 x 3; or 3 x 1
callsigns, as appropriate;
4) send two 'K's at the end of each transmission;
5) only send the outgoing report (i.e. do not re-send the incoming report),
and send it at least five times at each attempt; and,
6) only send a series of 'R's when you are sure that you have copied the
other station's callsign - and your incoming report - correctly.

In addition to the above : if you have a weak-signal sked it can be very
usefull to agree on fixed transmit / receive periods.
eg : stations A will transmit during the odd minutes, station B during the
even minutes. Of course you can also arrange 2 minute periodes (or even
longer).
At least you avoid that you will be transmitting simultaniuosly.

73, Rik  ON7YD