I have not tried this myself , but got it off an extreme couponing facebook page. If you go to tesco to buy something that is on offer and there is none left on the self, if you go to the customer service desk and ask them for a raincheck voucher they will print you off a voucher you can use at a later date to buy the item at the offer price even if the offer has ended.

also if you go to www. supersavvyme.co.uk  which i think is run by procter and gamble you can print off so many vouchers per month to use at tescos particulaly good for shampoo when herbal essences is on 2 for one offer as you get 2 bottles for around £1