Maintenance and emissions
A well-maintained vehicle pollutes less, uses less fuel and lasts longer - the three are the same problem seen from different sides.
Service at the recommended interval. Change the engine oil and the air filter on time: a choked air filter makes the engine run rich and smoke.
Keep the tyres at the right pressure. Under-inflated tyres increase rolling resistance, which burns more fuel for the same journey.
Fix a smoking exhaust rather than waiting for the pollution test. The test is a check on maintenance, not a substitute for it.