Power supply noise and ADC
The points you mentioned are just hacks. If you want a stable power supply for the ADC then you're better off using a voltage reference IC. They will provide an accurate and stable fixed voltage, with the drawback being that they supply current only up to about 100mA typically. The good ones have temperature compensation as well.

Eg: I use this one for 5v http://www.digikey.com.au/products/en?keywords=LT1461DHS8-5%23PBF
... and this one for 3.3v http://www.digikey.com.au/product-detail/en/texas-instruments/LM4132AMF-3.3-NOPB/LM4132AMF-3.3-NOPBCT-ND/1206403

You just need a couple of generic caps to go with them. Almost too easy.