Don’t open with your best offer. Even if it really is your best offer, it is all but impossible to convince an adverse party that it is. One and done almost never happens and opening with your best offer leaves you no room to negotiate. Once your offer is rejected, attempting to go backwards, i.e., to offer less than you have already put on the table, simply won’t work and will crater the mediation. So have patience. Make your first offer a plausible anchor and then negotiate from there towards where you need to be. Don’t make your best offer your first offer.