Rockandrollcows,
I use cheap brushes myself and to prolong it just a bit longer, I soaked the brush with acetone after use. Mind you, you'll have to soak it and clean it off with a rag. That means straight after use, keep 'washing' it in a small container and drying/cleaning' it off with a rag. It'll keep the bristles soft enough for reuse again.
Point to note, DO NOT leave it in the acetone container as it will harden up. Trust me, I made that mistake and had a nice brush sculpture after.
Hope this helps...if you need more info, LMK.
Stay safe,
Tim.