The material of the capacitance matters too, Try using an OSCON capacitor instead of electrolytic.
what frequency is the clock? the bypass capacitors should be placed as close as possible to the VCC/GND
pins, and the value of the capacitor is dependent on the clock.
1) Add a ferrite bead or choke to the power supply source to help filter out the AC
2) You can't rely on the internal RC oscillator if you need a precision clock. if you must use
an external clock/crystal, use series terminating resistors to reduce harmonics on the oscillator
3) Run the traces close together as the crystal signal is like a differential pair, since the output
is 180 degrees out of phase with the input of the crystal, they have cancelling magnetic fields
making it immune to generating or receiving EMI noise.
4) Study books such as 'electromagnetics explained' (on this forum), howard johnson, eric bogatin, Lee ritchey, etc..
are all good authors to help deal with these subjects.
5) Read articles such as the following by Douglass Brookshttp://www.ultracad.com/article_bypass.htm