How do I keep a longer lasting heat?
I'm going to assume that you are smoking with charcoal as electric smokers don't really have trouble with fluctuating temperature.

Plan on needing to add charcoal every couple of hours during the entire smoking period. I start mine at unwholesome hours of the morning and set alarms to get up and stoke the fire. It is nice if you can stoke the fire without letting out all the heat but that depends on the shape of your smoker.

Keep in mind that you only need to add your smoke producing wood for the first couple of hours. After that all you need is heat, no more smoke flavor will be absorbed.

It is very helpful to use high quality, lump charcoal. Lump charcoal burns longer and produces much less ash. Charcoal briquettes are filled with clay. Not only do they burn out more quickly but they leave mounds of ash that you have to clean out several times during a 10 hour smoking. Around here I can find lump charcoal in large bags at the hardware store and sometimes at Costco for much cheaper.

Another alternative:
If you aren't a purist who needs to be able to say that the turkey was cooked only with coal, you can always finish it in the oven. I've read that meat will soak up all the flavor that it is going to in the first hour and that matches my experience. After that it is just slow cooking which is much more conveniently done in a low oven.