I managed a charity shop for many years, so here's the best advice I can give. For electrical stuff phone up and ask as some shops do have the resources to check them, but if they can't they'll have to pay for the disposal. Clothing I'd give in whatever condition as the fabric can be recycled (Oxfam have their own recycling plant for unsellable clothes called Wastesaver in Huddersfield which makes a lot of money). Specialist clothing such as cycling kit should be bagged separately with a note suggesting that they could get good prices, often the sorters don't have the knowledge to make that judgement if they're unfamiliar with sports kit. i remember once donating a pair of Sidi shoes to my own shop and the volunteers priced them at £3.99, I repriced them at £30 and we got a sale immediately. Bric a brac items should always be in excellent condition otherwise they are useless and time wasting for the shop. Most of the weekly rubbish we threw out was unsellable bric a brac. Again with specialised items it's appreciated if you can label it so that the shop workers know that they may have something of value. Books too need to be in good nick or they'll end up in the bin.
When you take your stuff in it's worth telling the staff of anything of interest. They'll appreciate the help (but don't get out each individual item to show them at the cash desk, that'll drive them nuts

)