For racing spacecraft, you'll need to focus on conservation of delta-V (total available change in velocity, AKA: Total engine burn-time multiplied by acceleration) to allow for course-corrections (because space is big, and there's a lot of crap in space that can cause solar-windage and drift off-course). An interesting bit of physics is to pretty much power-dive at a planet at full-burn to improve delta-V (as you are shedding mass while gaining momentum) and aerobraking to cheaply bleed excess velocity so you don't land at your target-window like a bird from a windex-commercial.

Also, a lot of things can kill your delta-V rating, stowaways, particle-buildups, [url=http://www.universetoday.com/62672/13-things-that-saved-apollo-13-part-5-unexplained-shutdown-of-the-saturn-v-center-engine/]engine damage/malfunction[/url], and propellant-leak to name a few. Oh, and collisions hitting stuff at nearly the speed of light is bad... like, one kiloton of TNT of impact for every gram that hits you.

This said, about 90% of your ship's mass will be propellant, not living quarters, and not structural components, [i]just propellant[/I]. Your ship will accelerate exactly ten times better when empty than full.

The other issue is acceleration, as you may need to change your velocity 'quickly', and by that, [url=http://www.universetoday.com/62663/13-things-that-saved-apollo-13-part-4-using-the-lm-for-propulsion/]I mean precisely and if you cut the burn too soon, or start too late, you'll crash and die like someone who took a second too long to turn the wheel during a powerslide into a rail.[/url]