quick charge but slow discharge of a capacitor
Put two resistors in series with the cap and then put a diode in parallel with one of the resistors.  This will give you a different time constant depending on if current is flowing in to or out of the capacitor - one resistor will be bypassed by the diode in one direction.  Another possibility is to use two resistors in parallel, with each resistor in series with a diode, and the diodes pointing in opposite directions.  This gives completely independent control over the time constant since only one resistor is used at a time.  







simulate this circuit &ndash; Schematic created using CircuitLab

This may also do what you want, considering you have two distinct current paths:







simulate this circuit