The Shield is a good first upgrade due to its low cost, and should be upgraded twice as soon as possible. Also get the two Cape upgrades because longer gliding will give you greater control over your runner. Boosts such as extra lives, Bull, and Hermes should be done later in the game when they are needed. Purchasing the extra life always costs 200 coins, while continues cost 300 coins. It is usually better to purchase continues, since they will not be used until needed, while an extra life may be wasted if it is a bad run.
Cape
If you lose your cape, you can get it back. The level of your boost will determine how long this will take. The best way to increase your chances of getting it back is to ride a sheep as long as possible.
Controls
Short taps give you lower jumps, and longer taps allow you to jump higher. Gliding will usually get you as far as possible.
Helmets
Helmets are only cosmetic, and should not be purchased before more useful items.
Obstacles
Instead of focusing on jumping to collect coins, concentrate on the obstacles. Plan your run to always avoid them. Many coins are placed so that you will hit a fence, and runners will sometimes trick you into thinking that you will get past a fence sooner than you actually will.
Sheep
Always try to ride a sheep, as it is basically an extra life. The same goes for the shield power-up. However, you cannot glide when riding a sheep.