In my younger days I may have placed a few stickers on my bricks. Including some that covered multiple bricks.
Now my daughter has the tendency to do the same thing :)
How can I best remove the stickers, particularly old ones?
the best way i found out till now is to get them of as good as possible with your nails or a plastic-knife (i wouldn't recommend to use metal knifes or razorblades - if you do, be careful to avoid scratching the bricks surface) and then remove the remaining glue with:
I performed my own experiments to determine which methods work by randomly applying three stickers (which are similar to the stickers that come in LEGO sets) onto a simple brick wall.
I haven't tried hair spray, pure-alcohol or any other chemical cleaning products.
Best to remove as much as you can by mechanical means (plastic spatula/fingernail, nothing harder unless you like scratching your bricks) then use some 'Sticky Stuff Remover' to get the sticky residue off.
This stuff works on just about anything without leaving a mark, Lego is made from ABS and will not be affected by it.
If you don't care about preserving the old sticker, remove it with your fingers or a plastic knife and then use rubbing alcohol to remove the remaining sticky bits. As other posters have noted, metal knives/razors leave the risk of scratching the plastic bricks - it all depends on the level of confidence in your knife/razor skills.
I use alcohol instead of harsher chemicals (like glass cleaner or hairspray) to reduce the amount of possibly toxic chemicals on the bricks, since kids will be playing with them after the sticker is removed.
I use a hair dryer to heat the sticker up a bit. Then a plastic scrapper or plastic knife or fingernail to scrape off the sticker and as much of the glue as possible. The genuine LEGO stickers seem to come off better than the after market stickers. I use a bit of rubbing alcohol on a micro-fiber cloth to remove any residue. The remove any alcohol with a dry micro-fiber cloth.
This leaves nice clean LEGO's.
There is/was a product called Un-Du that would soften the adhesive of stickers so that they could be removed and reused. I was able to find a few links online, so I guess it's still around. The "patented" part of the "Un-Du" product was the tool extension attached to the bottle. The important part is the liquid and that is plain heptane. I haven't checked it against ABS, but I think it's worth a look see