My bike is going to be an Agattu.
I would like to add another consideration to the points by flecc. Use the power from your own legs at a fairly steady constant rate. Certainly, when using legs for running, endurance athletes working near the limit of their endurance tend to find that a rather constant rate of work ensures a shorter time over a given distance. Thus a roughly constant speed when running is the aim. If one judges ones capability wrongly and sets off at too fast a pace the runner will "die" or "hit the wall" and slow dramatically before the end. Equally a slow pace at the beginning may leave one with a lot of capability at the end for a sprint finish or in the case of an electric bike, energy to continue pedalling when the battery has run out- which might actually be useful!
In preparation for the arrival of my Agattu I have been try to determine what my steady state long term power output capability is. I think it is around 70 to 80 watts. (See
http://www.pedelecs.co.uk/forum/electric-bicycles/7512-wattbike.html?highlight=wattbike#post94455) My idea is that I should try and contribute 70 to 80 watts continuously and then call on the bike in its different power assist modes as and when needed to help me uphills or into the wind, or if I just need just that extra trickle of power in the eco mode to keep me up to my target speed, which for me is you may tell from my user name is a modest 10 mph.
Whatever your overall speed requirements are, keep the load on yourself fairly steady so that you can contribute the greatest possible number of your own watt hours before your battery's supply of watt hours is used up and both you and the motor grind to a halt simultaneously !