buy the T-10 ticket,you can use it 10 times in a month.it's really worth it esp if you stay for 3 days or more.straightly buy it the moment you arrive at airport-T2. It's much worth it then using a shuttle bus .and if you change train line for metro or renfe train,it's still consider as 1 journey.
Walk instead of taking the bus. It saves you money and helps you discover the real Barcelona. Also make sure you get a hotel with breakfast included. It keeps you going till the afternoon and saves you looking for food right after you wake up.
you can discuss the prices with the shop sales assistant and have great discount, but first compare the prices, most restaurant are cheap but overcharge the drinks! so always ask the drinks carte to see the prices
Buy a T10 bus ticket and a 2 day ticket for the hop on/off bus
Buy tickets to cathedral on line to save queues