You've asked a very entertaining question, and the answer is not simple.
Let's ignore collisions for the moment. The "purest" effect, that is, the one which involves no change on the part of the planet or its sun, is the effect of tidal bulges in the sun. Just as the earth, for instance, is not a perfect sphere due to tidal forces, so the sun is not a perfect sphere, due to tidal forces caused by the earth. The resulting bulge in the sun lags behind the planet, and essentially acts as a brake on the planet. Over time, the planet will gradually lose velocity, and will eventually fall into the star. For most planetary systems, the effect will take a very, very, long time, since the planet is much smaller than the sun, and far away.
But there's another factor to consider. Any star produces a "solar wind" which causes it to lose mass. The amount lost per year is small, but it never stops. The result is that, over billions of years the planet's orbit will grow larger as the gravitational attraction to the sun diminishes.
Finally, for stars like our sun, stellar evolution will eventually cause the star to become a red giant. If the diameter of the star exceeds the orbital distance of the planet, of course, the planet will be vaporized. Even it if doesn't, the tidal bulge will become much more effective in slowing the planet, and depending on details of the planet's orbit may or may not cause the planet to drop into the star before the star shrinks to red dwarf status.
In the case of the earth, according to http://arxiv.org/pdf/0801.4031v1.pdf that is exactly what will happen to the earth in (roughly) 7.59 billion years. It's notable that if the earth's orbit were 15% larger it would be safe. But just before the sun reaches peak diameter tidal forces conspire to slow the earth down and it plunges (will plunge)into the sun.
As for other considerations, such as explosions, impacts and shock waves, the answer is that they can have an effect, but generally not much. Basically, if the impact or whatever were big enough to make a major change in the planet's orbit, the planet would cease to exist, and would be replaced by a great big debris field. To some degree this would recondense into a smaller planet with a different orbit, but it wouldn't be the original one. Just as a thought experiment, though, if the earth were to hit another earth-sized body exactly head on, and the other body were in an identical orbit but going the other way, and the two planets fused instead of turning into a massive debris field, the resulting fused body would drop straight into the sun.
As for a planet ageing, for earth-types the answer is, not much. It's true that our kind of planet can lose volatiles such as water and air (and do so at a very low rate), but the total effect is miniscule. We are, after all, mostly rock and iron, and those just don't go anywhere. For gas giants like Jupiter, if they are close in they can get their gasses blown off until there is nothing left, or only the non-gas core. However, any such loss will be at right angles to the orbital motion (for more-or-less circular orbits) and will have virtually no effect the orbital motion of the planet.
Best Answer
As long as you add the mass in a way that does not affect its speed, then the orbit is not changed(your star must be fixed as well).
Lets say the planet(mass $M$) is orbiting at a radius $R$, about a star of mass $M_\star$. The orbital velocity is $$v_1=\sqrt{\frac{GM_\star}{R}}$$. Now, in the comments you stated that you added the mass in a way that does not affect its velocity directly. Simce momentum is conserved, the only way to do this is to give the added mass $m$ a velocity $v_1$ as well at the time it reaches the planet. As you can see, when the planet captures the mass, there is no change in angular momentum ($mv_1R+Mv_1R=(m+M)v_1R$). Now, since theres no change in angular momentum, it will orbit at the same angular velocity. If its the same angular velocity, the radius is the same as well. So it stays in stable orbit. One can get this directly from $v_1=\sqrt{\frac{GM_\star}{R}}$ as well.
What if the mass was at rest and it was captured? Well, then by conservation of linear momentum, the velocity would decrease to $v_2$. Since the velocity decreased, it will go into an elliptical orbit. If the velocity had increased, the orbit could be elliptical, but it can be hyperbolic (greater than escape velocity) and leave the system as well. This depends upon the mass ratio.
If the central mass was not fixed, then the masses orbit around the center of mass(barycenter), and the orbital angular velocity is given by$\omega=\sqrt{\frac{G\mu}{R^3}}$ (note that I'm using angular velocity in this case, as the star and planet will have different velocities). $R$ is the distance between the objects, and $\mu=\frac{MM_\star}{M+M_\star}$ is the reduced mass. One can see that a whole variety of things can happen, depending on how you add the small mass, and on the ratio between the three masses. You may want to analyse this yourself (seeing as it's not part of the question and it's a pretty interesting exercise)