Every other forum seems to have a sticky with tips and tricks. Lets see if we can start one here. From fitting mudgards / fenders to mountless bikes to good work arounds for expesive tools. I'll start off,
Zippie ties, toe clip staps and mole / quick grips help to give you that third hand.
Coke cans and similar make good shims.
Multi tools may get out of a tight spot on the side of the road but you need good tools. It makes life much easier. It is not wasted money as good tools will out last you. I use some tools from my grandads work shop.
To avoid cross threading nuts / bolts (easy to do with the fine threads used on bikes) you should always start off by running the nut / bolt in an anti-clockwise direction slowly (for a normal right hand thread nut / bolt reverse for left hand thread bolts) until you feel the nut / bolt 'click' / 'jump'. This is the point at which end of the threads on the nut and bolt have just passed each other and your nut and bolt threads should be fully seated against each other. Now gently turn clockwise and you should be fine and dandy if any excess resistance is felt back off check thread for debery and try again.
I will prob be shot down for telling people how to do something 'so simple' but it is something that a suprising number of people have a problem with when using fine thread and not poor quility low tolorace course threaded harwear store nuts and bolts. Hence the nuber of problems I have working on friends bikes when they have caused damage not through stupidity but through forcing things and not knowing.
If when repearing a bike you find damaged threads don't worry if the damage is light and you can find someone with one a thread file can clean up many a problem. (make friends with your local backyard engineer )
Lastly diesel is a brillient solvent for cleaning very dirty parts, chains and unsezzing parts. I was sold a nice racing bike cheap beacuse the gears would not work. Taken apart soaked in diesel for 24 hr the drive chain worked beautfully. (Note: avoid getting diesel rubber as it will cause it to perish)