Click here to auto-tweet in morning in the UK (9:30AM BST) and here to auto-tweet in the morning in the US (11:00AM CDT - I'm doing both!).
Also start thinking about what you'll do with your day. Are you near any events? Can you get to further away events? Is there anyone you would like to invite/go with to an event? Listings here http://www.bivisibilityday.com/year2015/ which will fill up with details as the day draws nearer. Good tip from this page - be aware of Jewish friends holding Yom Kippur on 23/9 this year.
I generally only have time for a Facebook/Twitter spree, which friends tell me is quite positive action - flood your feed with stuff and even with FB's algorithms, something gets through to everyone. As long as it's not in your face or angsty, no one wants that in quantity on a day of celebration. I try and paint my nails, wear my "Keep Calm It's A Bisexual" tee shirt, rainbow pin on my coat, and just find ways to mention it in conversation whether at work on socialising.