Do you have trouble fitting the nuts on the end of crankpins (e.g. Gibson or Ultrascale)? You can buy very thin fibre-glass pencils that have a small clutch at one end, which holds the fibre-glass stick and which is opened by depressing a button at the other end of the holder. I bought mine from one of the tool suppliers at Scaleforum (Shesto, I think). Discard the fibreglass stick, and the clutch can be used to grip the nut and act as a sort of spanner to get the thing onto the crankpin.
Of course, the advice is "only remove them when you need them". Fine, except that you always need the big bits first and the little bits are often attached to the big bits. When you need them, it takes ages to find them right? Simple answer is to visit your local stationers and buy a couple of hundred small brown envelopes (I got mine in packs of 50 - they are only about 3" by 2"). Number them consecutively, and when you have to detach a part for later use just pop it into the corresponding envelope until it is needed! Keep the "used" envelopes in numeric sequence and locating that elusive part is simplicity itself. Word of caution – empty all the envelopes when the kit is complete. There are often alternative parts so you may end up with the wrong bits for the next kit!
I find this especially useful for coaches where there are usually several handrails of the same size to be produced. Just take a piece of scrap brass (from the edge of a fret, for example) and drill two holes the distance apart being the length of the handrail, and the same diameter as the wire (usually .45mm). (In a kit, the holes are usually etched into the side - use these as a template). Drill one of the holes as close to the edge of the piece of brass as possible. Now file the edge of the brass until that hole becomes a notch in the edge. Bend a right angle in the wire, place it into the other (complete) hole, and form the other bend using the notch. Very simple, and they will all come out the same length.
Although I am not a smoker, I understand that the best Havana cigars are produced by rolling them on a Cuban virgin's thigh. Whilst unable to find a Cuban virgin, I do find the technique works well for rolling boilers (or any other curved sheet) but using my own thigh! Place the sheet of brass on your thigh and place a round bar of smaller diameter than that required for the finished piece squarely on the middle of the metal. Press firmly and evenly on both ends of the bar and roll gently towards you, then away. The sheet will curl into a nice even curve. Repeat if necessary to get the desired curvature - I find it better to make the curve slightly tighter than required for the final fit. I have successfully applied this technique to several etched kits where the sheet is very thin - for thicker sheets it may help to anneal it first.
The normal technique for decals (or waterslide transfers) is to place them in a dish of water before applying them to the model. This is fine for large ones, but can be very difficult for things like coach or wagon numbers. What I do is place the thing on the model and dab it gently with a paintbrush dipped in water. The surface tension of the water will leave the transfer floating on its own miniature lake. When it is ready to come off the backing, carefully soak up the blob of water with a piece of tissue being careful not to touch the transfer (capillary action will draw the water into the tissue). If you touch the transfer the letter is likely to attach itself to the tissue! Manoeuvre the letter still on its backing near the desired position and gently slide it off with the point of a craft knife or pair of tweezers for final adjustment. If you do not soak up the excess water, the thing will float of its own accord making positioning very hard! Incidentally, make sure the area for the lettering is a very glossy surface - they can be positioned more easily and will adhere better. A subsequent coat of matt varnish will disguise the gloss and protect the lettering.