How would I increase the power in an RF signal?

Using a crystal oscillator and some other components I made a weak AM transmitter. I'm interested in trying to increase the power in the signal so that I can transmit further. Before anyone asks, I'm nowhere near the legal power limit in my country for this frequency so am not breaking any laws.

I was just wondering if anyone could suggest what I do next, 1. as I said I'm looking to improve the power but it doesn't need to be a particularly long range (although I'm assuming power dictates the range it can transmit at?). I'm also keen to get it to work with a crude crystal radio circuit (just a diode, antenna and a crystal earphone)
2. is there anything I can do to my transmitter circuit to make it easier for a signal to be induced in the antenna - (it doesn't matter if it only works from across the room, so long as the signal is strong enough to induce RF in the antenna)?

you may find these threads as helpful:
https://www.edaboard.com/thread320829.html

And

https://www.edaboard.com/thread319685-3.html

Why thank you!
I wish you many Joules in your coulombs!

As you do not indicate the frequency you use, there are several points to bring to your attention:
The transmitted power itself is not the only important quantity. You also need a good antenna, well matched to your transmitter. And with a low power, you need a better receiver than a crystal set.
I would recommend you to contact a local radio-amateur group. Radio amateurs do communicate around the world using short-wave bands and often low-power (QRP) transmitters. Find the ARRL Radio Amateur's Handbook where you can find many technical solutions.

use high power amplifier for the last stage. design its driver stage/s and link current transmitter output to the driver

Woah, I'll check these out! Thanks!