Updating my plugin without releasing a new version

yes, it’s absolutely possible.

When you’ve done developing your plugin, you set Stable Tag field on trunk/readme.txt to, lets say 1.0.1
This value should match with Version field on your plugin’s meta data.

Then, when you publish your plugin, you tag it with, let’s say, /tags/1.0.1/

To continue development, change Version field on your plugin’s meta data to something else. Increment it would be the better choice. But, you don’t need to change your readme.txt. Leave Stable Tag intact, just change it once you are ready to publish it. And follow the previous step.

Note: This information is available on wordpress’s page too.