Power and ground as close together as you can get them, preferably "stitched" together with 0.01uF to 0.1uF monolithic caps at every IC. Signal path short and direct. Minimum trace length is always correct.
what should you do to be "careful" and "lay out accordingly?"
1. know what points are high impedance points (i.e. FET inputs to opamps, FET gates, darlington inputs, bootstrapped bipolar inputs...)
figure out what the input impedance of each place a signal possibly could come in through capacitive coupling. If you don't know this, within at least an order of magnitude, you are not being careful.
2. Lay out accordingly by making it hard for capacitive coupling to happen. Capacitors are made higher capacitance by making the plates bigger and closer together. So to separate things, you make the voltage carrying places smaller and further apart. If you can't move them apart, put a grounded piece of metal between them to shield them. I use guard traces all the time.
3. Know and use grounds properly. Ground is not ground, as I'm always preaching. Ground may be (a) safety ground (b) voltage reference ground or (c) sewer ground. You must be able to point to any ground trace and say which of these it is. Mixtures are bad.
However, on tiny circuits, this may all be wasted. The smaller and more importantly lower gain the circuit is, the less sensitive it is to cross coupling. One transistor circuits can get away with murder. Super high gain FET distortion pedals with many stages ARE murder.
