Can't be bothered reading all the replies so sorry if they have all been mentioned.
A bus from the airport to the centre of town is less than 10% of the cost of a taxi so depends how far your hotel is from the centre of town.
If using a taxi, agree and haggle a rate before getting in and setting off to town. Savings to be had. This advice applies anywhere not just Prague.
Try and find a local czech pub/restuarant and not one made for the tourists for a feel of traditional food and cheaper prices. Last time I was there (3 years ago, 6 of us had a meal each, 6 pints of lager, one vodka chaser, two packets of fags for £28

which was literally 400 yards down a side alley from the astonomical clock where six of us had just had 6 pints plus one vodka chaser for the same money sat in front of the clock at a tourist bar)
Keep your wallet in your front packet of trousers/jeans. Never in the rear or the inside jacket as pickpockets are rife. Only take the money you need out each night plus one card max for emergencies. We met loads of people who had their wallets lifted while we were there including one of us. I wouldn;t mind but a guy on the plane gave us this advice and all but one of us took it on the first night. He had his wallet in the rear of his jeans and in one bar it was lifted. Worse it had all his money for the trip, all his cards, his driving licence and even his airport parking ticket!
Language no problem as 95%+ speak English.
Trams are cheap to get around just make sure you always buy a ticket (6p for inner circle trips when I was last there so no excuse really) as the fines are huge if caught without one.
There is a discount card which lets you into art galleries, museums, tourist attractions all cheap or free if that's your thing or you are there long enough.
That's about it. Have fun