Winnie, My toddler is exactly like this. It is hellish, isn't it?
Practical tips:
Yes, food - go for things that take a long time to eat. Crisps and munchies - I used to get the Boots toddler versions. Also, an ice cream, though the disadvantage of this is shop assistants will frown at you. I used to take a plastic pelican bib with me until my toddler could eat an ice cream without too much mess. Less messy - jelly sweets.
Check the straps, can they be any more secure - but I bet you've done this already!!
Buy a cheap toy when out - a desperate measure again, buy a £1.00 toy car. My son examines both the toy and the packaging, and this can buy me half and hour of peace.
Dress your son to wear bulky clothing - it helps jam him in, making escape bids more diffucult. This in on the assumption that your son wriggles out of the straps, rather than undoes them. Last summer I could hardly keep my son in his pushchair, then the winter came - he was dressed in more layors and suddenly my life became tons easier.
This summer I have been thinking of hunting out a padded sleeveless jacket. The weather is cool enough for this.
Last tip - I put him in a shoppers creche whenever possible. An hours fee isn't far off the amount I would spend on toys, crisps and drinks.
Must dash because we're off shopping this morning - eek!!!