Keeping connections accessible, joints aligned and sealed properly, pans matched to cisterns, and appliances fixed so nothing leaks.
Keep connections accessible
Fit trap and branch connections, and rodding access at bends and junctions, so a future blockage can be cleared by removing a trap or opening an access point rather than by dismantling pipework or cutting into tiling. This applies as much to a basin waste tucked behind a panel as to an exposed pipe run.
Align and support every joint
A WC pan connector - or any similar mechanical joint - must sit square and be properly supported. Forcing or twisting a connector into place strains the seal and can leak foul water or restrict the bore. If a joint like this starts to weep, remake it properly; sanitary sealant is a finish for a sound joint, not a repair for a failed one, because water tracks behind a bead and the leak reappears somewhere less obvious.
Match the pan, cistern and pipework to the appliance
A WC pan is designed to clear fully with a particular cistern flush volume and pattern. Pairing a pan with an incompatible cistern can leave waste behind or force repeated flushing. Waste pipe material matters too: some plastics soften or distort if the discharge is hotter than they are rated for, straining joints and causing leaks, so the material has to suit what the appliance actually discharges.
Fix appliances securely
An appliance that rocks under use - someone leaning on a basin, for example - strains its supply and waste joints until they weep or fail. Fix every appliance firmly enough that normal use cannot move it.
Washing machines: use an open, trapped standpipe
A washing machine or dishwasher usually discharges into an open standpipe of adequate height fitted with its own trap, rather than a sealed direct connection. The trap blocks drain odours as any trap does; the open air gap where the machine's hose enters the standpipe stops drain water being siphoned back into the machine if pressure in the branch drops.