-Use a supervisor chip, not the RC reset. What is it?.
http://para.maxim-ic.com/compare.a...rs.cfm&ln=

-Put 100nf capacitors directly across all chips...
what all chips?
ALL chips whatever they are, IMPORTANT, do it now

-Put a 4.7 or 10 uF tantalum directly across the '51.
accross what? Vcc and GND of micro? I have a filter now...
filter? describe

-Use a 4 layer board with ground and Vcc planes.

-Use differential serial communications (422/485). N/A, I guess

-Put an VDR at raw power input. (What is this?) Voltage Dependent Resistor. This will cut all spikes on the power line

-Opto-isolate all non-differential signals.
May be a good idea for the signal from the flowmeter.

-Put ferrite beads on all wires at the place where they enter the box.
important for your inductive load. Also, have snubbers across it

-Do not drive capacitive loads (like ribbon cables) from CMOS outputs...Do not have this in my design.

Erik