If you're stuck for new XP-earning activities to take part in, refer to the little book icon that appears in the bottom left-hand corner of the screen. Your trainer will always have a steady stream of new quests for you to take part in, and while the XP gains may not be massive, this constant trickle of experience soon adds up to new levels.
Not all items are created equal when it comes to the amount of XP you get in return for the amount of money invested. Before spending you hard-earned cash, take a look at the items available to you, and pick those that will give you the most bang for your buck. The more efficiently you spend your money, the more opportunities you'll have to level up in a play session.
What's good for making more money in Clumsy Ninja is good for gaining more XP too. By this, we mean you should become as proficient at the gameplay as possible, to minimize any downtime to your ninja. Get those early trampolining and punching bag mini-games down perfectly, so you eliminate any stumbling moments from your XP opportunities.
Just because you can't earn any more money from mastering an activity, doesn't mean you can't continue leveling up further. If you've found a cheap source of XP, by all means keep up the activity to get you closer to a new level. Again, make sure though that you're getting the most XP bang for your buck.
Always experiment with your little friend to see if there are any new XP opportunities you haven't tapped into yet. Fling him around, shake his hand, play with him nicely - just keep a note of what boosts his XP, and add them to your daily list of activities so you're leveling him up as quickly as possible.
