Throg's first rule of repairing is never ever use the "Repair All" button. This is a slick sales gimmick by the trader to try to get you to repair stuff you don't actually need. After an evening adventuring your bags are likely to be full of dented weapons and armour which may fetch a few shillings from a vendor. Clicking repair all will rack up a hefty bill for fixing these BUT generally the cost of the repair is more than you will get from selling the repaired item. Never repair something that you intend to sell soon - it just isn't worth it.
Throg's second rule is to be very choosy about the magical tems you carry and equip. Magical items incur a very hefty repair cost and unless they convey substatial benefits may not be worth wearing. A case in point was an earing of increased will which Throg sported for some time. Will increases an adventurers power reserves - nice by not essential for a champion. Throg realised that this one earing was costing several silver pieces on each visit to the repair shop so he decided he could live without it. It now resides in his bank vault and our dwarf has not noticed its absence.
Throg's third rule is not to repair items that have more than 25 points of durability. An item is not going to lose this amount of durability in one encounter so you will get a chance to repair it later if really need be. More importantly an adventurer will inevitably find better equipment in his or her travels and leaving repairs till the last minute increases the likelyhood of finding a replacement. Throg makes an exception for his gathering tools because you never know when a profitable ore node or pile of wood will turn up and it would be a shame to lose out on account of a broken tool.
Throg's fourth rule is to consider replacement rather than repair. It is often possible to buy a better item on the auction house for not much more than the cost of a repair. The disposable age has come to Middle Earth and Replace rather than Re-use is the new Mantra.