What to do if drugs / a bomb are planted in your travel bag?

Best decision depends on the circumstances, including reliability of the police/judicial process, your access to a good lawyer, and your personal situation.

If you're a reasonably wealthy, respectable-looking professional traveling to the U.S. or Western Europe, your best bet is to alert the authorities immediately, not make any extensive statements, and contact a good lawyer.

If you're a backpacker arriving to, say, Southeast Asia, disposing the contraband before customs (for instance, by flushing it down the toilet, or leaving it somewhere hidden, trying not to leave fingerprints or be seen) seems like a better option. If you report it, the police will be interested either in a sizable bribe or to show off a flashy conviction without doing too much work. You're at hand and make a good target, while apprehending the real criminals may be difficult to impossible. The real criminals may even be paying off some of the cops (or they could be senior cops themselves). Once you successfully dispose of the contraband, I think your next priority would be to turn right around and get out of the country ASAP.

If you notice your luggage has been tampered with before customs inspection, immediately alert airport security/police/customs agents (whichever is closest). What happens after is impossible to tell. Most likely at the very least your bags will be confiscated as evidence in a criminal investigation. You may also be detained or even arrested at least for the purpose of questioning you (and you have brought potential contraband into the country, which might be cause for arrest even if you voluntarily hand it over in some countries).
Don't just try to sneak through customs anyway and get rid of it later, or flush it down the toilet before customs.

As a person who travels at least once a month all over the world, I have noticed something, in all airports there must be a toilet somewhere between the airplanes gates/carousels and the customs/immigration counters. The easiest way will be disposing things in the that toilet (if possible) specially if you are in a foreign country and you are not sure of the laws there, some countries' officials will not be so understanding and will treat you as a suspect immediately so you better get rid of the stuff right away and continue with your journey problem free. Second option will be (Hold still, Call for official help). There is one exception, If you find a bomb I believe you should do one thing: RUN AS FAR AS POSSIBLE WHILE SHOUTING FOR HELP.

Any way, Please always make sure you check your bags yourself once you reach the airport and before you check them in. Always use some kind of lock. These little efforts might be the best solutions to these problems. Always keep your bags at your sight and never ever leave your bags unattended.

Drug smugglers don't plant drugs in innocent people's bags. They have a steady stream of willing volunteers and it is much easier for them to collect the drugs at the far end when they have been carried by paid mules rather than innocent bystanders who might discover the stash and alert the authorities. The drugs are worth a lot of money to the smugglers and they won't take unnecessary risks with them.

Great question. Most airports have rubbish bins. If you "find" some contraband in your luggage, throw it in the rubbish and keep moving. If there are no rubbish bins, consider using the toilets and flush it.

Even if you get caught, you can at least explain legitimately that you did not intend to bring it into the country, which may or may not help your defence depending on which country you're in.

There's a reasonably decent American mystery novel that features a courtroom scene with the defense attorney asking the Court to weigh the evidence. (The defendant is charged with smuggling.) The judge replies that he always weighs the evidence, but the attorney means literally. Without the drugs, it matches the weight on the checked baggage receipt. With the drugs, it doesn't.

I would not rely on Perry Mason to get you out of such a mess, but who knows!? (Alas, this is too long for a comment.)