wear comfortable shoes, tough walking ones, as the cobbled streets are uneven and there is an awful lot of walking, make sure you pack cotton socks too.
If you want a taxi - beware! Lots of them are operated by the Mafia - they'll give you the run-around, and/or take you to clubs and pubs where they get a commission, and duff you up with the help of bouncers if you object to the rip-off fare. Don't just flag one down. Book through your hotel desk, or other establishment you know to be reputable.
as dothawkes said - good walking shoes are advisable. If you go to the astronomical clock you could meet up with one of the walking tours - we were on this for about four hours which sounds a long time but was a thoroughly enjoyable way to see Prague. There are lots of lovely restaurants but unfortunately i cannot remember names - down by the river and in the centre. It is very enjoyable just sitting in the main square watching the world go by - perhaps sipping a cool beer surrounded by all nationalities!! You will thoroughly enjoy it - everyone we met was very friendly. I am not aware of any donts. We were only there a couple of nights and felt once we had done the walk we were free just to wander on our own. If I go back I would go on a river cruise.
get a pass for the Prague transportation system (bus, tram or metro) - passes available for 1 day, 3 days or 7 days & cost 80 Kč, 220 Kč & 280 Kč respectively. The system is said to be one of the best in Europe.
Don't let your partner out of your sight near Wenceslas Square. My husband (early 60s) and our friend (early 70s) were approached by a gorgeous young girl. It did them a power of good....just the thought, no action!!