For example: You're sitting at a table in a pub with an empty glass in front of you. Instead of having to take the glass to the bar to ask for a refill, wouldn't you be more likely to buy another if a member of staff asked you if they could get you one (having spotted your glass is empty). That's being proactive, rather than reactive. You'll sell more drinks as a result, to happier customers who feel they've been looked after, and will be more likely to come again, and bring their friends.
Check out this brilliant idea by a car dealer in Brazil:
Their demo car follows breakdown trucks around town to offer stranded drivers a chance to get to their destination whilst they experience (self-evidently) a better car than the one they own - all free of charge. The breakdown company probably also make it look like a part of their own service - and will therefore alert the dealer to every call-out. Everyone wins.
What about applying this proactive philosophy to tyres. Send a kid armed with a tyre gauge and a small printer attached to a smartphone, into a large car park. He's looking out for tyres with illegal or nearly illegal tread depth. He measures them with his gauge, enters the car's reg into the device (which then knows what tyres the car needs) and prints out a simple note that says: "Did you know your tyres only have mm of tread left. The legal limit is 3mm. Unfortunately your car is in a dangerous condition and you can be fined £1000. For just £99 per tyre we'll bring a mobile fitting van to you. Call 0800 DIALATYRE now, before it's too late." Why wait for the customer to make the decision to buy some tyres? We always put this off. So instead of advertising (expensively) something like: 'We sell cheap tyres, come to us at...', you've not only found people who need you, and need you NOW, you've also given them a quick and easy way to satisfy their need. No hassle, fast service, and impressive.
Once you've got the hang of the Proactive Selling concept, you should be able to come up with an example for practically everything anyone sells. It's all about thinking about what and when people might need things, and then working out how best to approach them with a proposition. It's a little like ambulance chasing, but smarter.
Salesmanship: You're either lazy and wait for marketing to drive customers to you... or you get off your arse and find them.