One thing that I've found hugely helpful is that my daughter loves getting post. Although we don't see each other much during the week I've found that a postcard just lets her know that I'm still thinking of her (every day, although I don't send a postcard every day!).
Even if I'm only in London, it's still "Daddy on a trip" and so an excuse for a postcard. She loves receiving them, and even encourages me to head off on my trips so that I'll send more! I've managed to turn a difficult fact of my life into something fun for us both, and hopefully one day in the future she'll be able to look back at great memories of our weekends together, and great postcards that will inspire her too to see some of this crazy world.
Being a single parent is incredibly hard and difficult, and being a distant parent is very tough too. Hopefully this little thing that works for me and my daughter might inspire others to find something similar.