100 pushups is a noble goal, but if your max is more than 10-15 pushups with good form (without increasing resistance with weights / elastics etc), you're going to be building more endurance than absolute strength / explosiveness, and size.
You can increase your endurance by doing more reps of an easy exercise (mainly slow twitch fibers) or by doing harder exercises to increase your absolute strength (building fast twitch fibers). Building your endurance doesn't increase your absolute strength as much.
If you're traveling, have no access to equipment, you can still change up your workouts so you're building strength and explosiveness. As an approximate rule of thumb, you should be doing exercises where you can't do more than 10 reps - this way you're building absolute strength, rather than endurance.
Try the following progression; if you can do 10 or more reps, make it more challenging:
Regular pushups (hands ~ shoulder level) --> move hands lower toward belt and outwards (think similar to dumbbell flies) - you reduce the leverage to make them more challenging. You'll need to turn your hands outward with your arms
Same thing for diamond pushups - start with hands at shoulder level, move downwards toward belt
The book Building the Gymnastic Bodies is a fantastic book on body weight exercises, including push-ups. Of course, I do not condone illegally downloading the book regardless of how easy it is to find as a PDF - find it at
http://www.gymnasticbodies.com/