Don't flush sanitary towels or condoms down, don't use bleach or toilet blocks to clean the toilet; use white vinegar instead. Try to avoid using soaps, shampoos and detergents with high amounts of surfactants in them. Pick up the odd ripe road killed corpse and throw it into the septic tank every now and again to top up the bacteria levels: for some reason a dead cat is absolutely perfect for this.
We pumped our own septic tank out once a year in the autumn, normally just after the first frosts. The vegetable patch was 30 metres X 10 metres, and this we ridged with the potato ridger attachment for the cultivator. The submersible pump we used was rated at 1000 gallons/hour (or there abouts), the first 15 minutes worth was pumped on to the lawn, the next 15 minutes worth was pump into the bottom of the hedges, and the rest was pumped into the vee shaped furrows of the veg patch. Where it was left over winter before rotovating it fully into the soil just before springtime sowing and planting.