What is the best way to protect frame from dents during crashes? I have a bike and I crash relatively often. I'm concerned about protecting the frame from damage.

I started searching for a way to resolve the problem and I'd like to share options that I've found, discuss their pros and cons and maybe someone would share their experience and other options to protect a frame.

Protective film



I discovered that there is a such thing as a thick protective film that can prevent minor-to-medium damage to the frame's surface. There are special film kits for bicycles, but they are expensive. I'm going to use industrial protective film which is cheaper and probably of greater quality.

Derailleur guards



It's unlikely that I will use them because during crash they may work as a lever and inflict serious damage to the frame.

Adhesive bumpers/pads

I've found special pads for bicycle:


But I didn't find where to buy them.

I also found generic adhesive bumpers:


I think I'll try them but I don't have much hope that they will work.

Motorcycle crash pads



I have a wild idea about fitting such devices to the bicycle frame. But these things also can lead to a serious frame damage.
Your bicycle is probably as protected as its going to get when it comes out of the factory, so I'd suggest doing nothing (well, maybe crash less). 


Protective Film -- Cosmetic protection (possibly structural in some cases for things like cable rub as pointed out by Nathan in the comments). If you're worried about keeping your bike shiny and scratch free I suppose. Be careful with Carbon Fiber depending on adhesive. Won't protect from dents. 
Derailleur guards are a terrible idea -- the force transmitted in a crash will be applied in a way not in line with design and damage the frame, whereas if the derailleur is installed as normal, the hanger will bend (or break) and is usually easily replaceable or reparable (align with something like the Park Tool DAG). If the derailleur is damaged, it's a heck of a lot cheaper than repairing the frame damage caused by a derailleur guard. You could spec an internal gear hub or single speed instead, if you want to avoid derailleur damage depending on the frame.
Bumpers -- See point 1. I see these as likely less useful than the film. 
Crash Guard -- See point 2. Plus, there aren't as many nice mounting points as on a motorcycle since you have to pedal on a bicycle for it not getting in the way, whereas you only shift+brake with foot movement on a motorcycle.