For officers I use the JIS rank system put forward in various places, including in Traveller's Digest:
O1-O3: 1 to three small silver "starbursts" (1cm)
O4-O6: 1 to three large gold "starbursts". (3cm)
O7+ 1 or more stars (IMTU they are USUALLY 4 pointed...)
The Starburst is the ring of fire around a circle...
Small (1/2sized) pips are worn on collar insignia.
For sleeve cuffs, use stripes of 0.5 for silver and 2cm for gold. O7+ get a 5 cm stripe, and have their stars .
O1-o3 IMTU also get silver leaves on brims, small display. (25% or less coverage), Brimless hats get 2mm silver cording/piping
O4-o6 get gold scramble, 25-50% coverage. Brimless: 2mm gold cording/piping
O7+ get multi-metal scramble of about 75% and piping (usually gold and silver, but marines often add a red metal)
For enlisted, I use a system of chevrons AND the "Dot Cross" by region. IIn the rimward areas, (former solomani areas) use the crosses.
For chevrons, I modeled after the 1870's US Army, Modern US Navy, modern russian, and commonweath, but hybridized to the following:
Recruit: none
Private: 1 diagonal hash, raised to back
Lance Corporal: two hashes
Corporal: Three hashes
Sergeant: 1 chevron
SSgt: 2 chevrons
PltSgt/Gunny Sgt: 3 chevrons
MSgt: 1 tripple width chevron
SgM: Tripple-width chevron with Starburst.
The chevrons are 2cm wide stripes, 10x7 cm overall, spaced 0.5 cm apart. and with a 0.5cm flash around (and between). Points are down. Naval personell have a specialty mark between points on flashing. Ground forces do not wear specialty mark.
The hashes are 1cm wide, 5x7cm overall, and spaced 0.5cm apart.
The Tripple is actually 7cm wide, 10x14cm overall, with flashing as above.
Stripe colors denote specialty. Flashing color denotes service. Hat and colar insignia are pins, and are pinned through appropriate flashing. Silver for naval ranks, gold for ground. chevrons on collar are 3mm wide, 20x13 cm...points worn towards front of collar, headband of hat.
Epaulettes, where needed, or breast-boards, are usually half-scale, with top of stripe at top edge of straight side.
IMTU, Retiree enlisted recieve pins with final insignia in service color on silver (space)/gold(ground), and may wear stripes of same pattern on dress uniforms. Officers use "hollow" starbursts on retiree insignia, color of service filling the ring of fire. All retiree insignia are issued with name, ID number, and dates of service etched.
This last I did to make the old russian practice of continuing to wear the uniform post-service possible without much confusion.
I have cadets wear gold or silver bars for cadet ranks, same pattern as real officers. Cadets in the field wear officer uniforms without pips.