There isn't a magic solution to reduce emissions of older cars, I'm afraid. The best thing you can do is make sure that you're engine well maintained (fresh oil, filters, etc), that you're using good quality fuel with a cleaning agent. If you're looking to improve the fuel economy, make sure your tyre pressures are correct and that you remove any unnecessary luggage from the vehicle (especially if you've got a boot full of spares or tools). If you're worried about the vehicle's performance at the MoT station, then book it in for a specific time and take it for a good 30 min drive beforehand to allow the engine to reach optimum operating temperature.