What 2slow said, I'll add to that.

Down thrust…The lift of any wing, increases with velocity. This should make sense without needing too much explanation. The faster you go, the more lift you make, hence eventually you take off and fly. Get too slow, and the lift decreases, the plane descends. Typically tailed planes have a nose down pitching moment, i.e. they want to tuck and drill a hole in the earth. To prevent this, the stab/elevator apply a down force which lifts the nose. Ideally, one would trim their plane such that the down force on the stab exactly counteracts the downward pitching moment, hence we have level flight. However, this perfect "trimmed" setup only works at one particular speed. Change speed and wing lift, pitching moment and stab down force change, and they don't change in equal proportions. Note: this nonlinear relation between these forces varies greatly across planes, esp wings. Without getting too technical with moments, etc., symmetrical wings are more behaved in this respect than flat bottomed/under-cambered wings. This is one reason why trainers with flat bottomed wings tend to “balloon” with velocity. Say you trim for level flight at 80MPH. Increase to 90MPH and you will climb, decrease to 70MPH and you descend. There is nothing really wrong with this and might actually be preferred in some models. However, it can be annoying, especially in aerobatic models. The idea then is to set engine thrust slightly down to aid the down force on the tail, based on engine thrust. This makes the assumption that more engine thrust equals more speed. There is a loose correlation there, but it should be obvious that it’s not a perfect relation. I.E., thrust on a 45 up line will be higher for speed X than speed X in level flight, etc. Nothing's perfect, but the general idea then is to set down thrust such that the effects of changing speed on elevator trim are minimized across the typical flight envelope. From this, it should be apparent that changing props will effect down thrust, i.e. 10x6 vs 11x5 for example, the 11x5 will produce more thrust than the 10x6 at X velocity.

Right Thrust…Beyond torque, prop wash on a single engine, single prop plane is asymmetrical. On a standard rotating engine, the wash swirls down the fuse, pushing harder on the left side of the rudder and fuse than the right. This causes a left yaw. There is a correlation between thrust and left yaw. By adding right thrust, the engine will counteract the left yaw. Again, nothing is perfect and right thrust can really only be set perfectly for one given speed, engine RPM and prop. The idea is to minimize the left law effect across the flight envelope. Like down thrust, changing prop will change right thrust.

As for changing thrust, placing washers on the mounting bolts between the motor mount and firewall as mentioned in prior posts is a common practice and works well. Some models will have the firewall offset to begin with. Others may use standoffs cut to varied lengths to create the offset.

Another note, you can simulate down thrust and right thrust, just as good if not better, with a computer radio. Simply mix throttle as master to elevator/rudder as slaves such that as you add throttle, the radio will mix in some down elevator and right rudder. It's not as strange as it sounds and I use this method on many of my planes, esp pattern planes, but I also often do this on sport models too.

Cheers