It is the little details in your scene that make the difference between a 'that's nice' piece and a 'wow, that is good...' piece.
Not the best picture here but this is a 25mm building, it was a lookout post, but it has some detailing points in it.
Moss, and grass bits at the base of the building, rust on the barrel, dirt wash on the boards, the window sill is weathered, and the adobe mix for the walls is not too big for the scale.
This photo I took at an angle trying to get a good picture of the barrel. As a result, the barrel looks larger than it is, (kind of like those pictures where you hang a fish on a line then stand back and hold out your hand (makes it look like your holding a big fish - the pre-photoshop days)
Anyway, if the barrel was really that large in relationship to the window and building it wouldn't look right.
Doors and Windows
Windows can be cut out of some things, 1/4" hardware cloth works good and someone told me the bottoms of plant trays (but I haven't seen any that would work). You can raid broken toys too but it is hard to get a lot of them the same.
I like coffee stir stick sized wood for windows, popsicle stick size is too big. Balsa is good but more expensive here.
For doors - tongue depressors work excellent. I like the curved tops for castle doors.
Besides using real metal and hammering it to shape. There are some wonderful new-ish paints on the market, they come in kits and in single bottles. So far the only place I can find them here is in the terribly over-priced Michael's store. Wholly crap how people can afford to shop there I don't know. But for the rest of the Serf's out there, they do come out with a 40% off coupon in their flyers; it is the only time I shop there and I usually only buy my one item.
Anyways, enough rantin'. The paint comes in various base coats, (Iron, Copper, Bronze, Gold, etc) You give your piece two coats with this stuff then let it 'cure' over night. Then you paint on a finishing coat, it is usually some kind of oxidizing agent. In the case of the barrel above, when painted over the iron base coat, it rusts. You can't beat the effect of real rust on your piece. "The painting is great, it looks just like rust."
There are some great painting techinques for painting a shiny metal effect. Basically, you play with the light, having it light-to-white in the places where say the sun would shine on it. If it, say a shield, was shiny and light was reflecting off it the bottom of the shield would be reflecting the dark ground (darker silver) and the top half would be reflecting the sky (silver-white).
For weathering wood, for window sills and fencing, I use a wire brush. There is a tool called a 'distresser' basically it looks like a stiff wire paint brush; but you can lengthen or shorten the bristles on it. I have made several, but I've only seen one for sale. They wanted $20 for it, I laughed. Like I said I've made several, the cheapest was bristles pulled from a wire brush with plyers and stuck in a plastic straw from a candy sucker.
If I am making a lot, i.e. a bunch of fence planks, I will use a wire wheel on a bench grinder or drill and run the boards (coffee stir sticks) over the wheel. CAUTION: finger skin loss is eminate!
Real weathered wood can be peeled from an old door or piece of plywood. I put nice looking bits of wood on top of my shed to go grey. Only takes a few months. Some bark mulch for gardens looks pretty good. Go look in the planters around the city.
For creating the red brick effect on my buildings I first carve out all of the bricks into the wall, either with a wood burner or with the hobby knife pencil method. Then I paint them all black with a paint-glue-plaster mixture and let it dry. Then I mix white clue and paint (red-oxide colour) paint all the bricks again sprinkle my red sand mixture on. I also sprinkle it and larger piecesof the sand along the base of the wall.
I use ground up lava stones for the sand.
I use the same red dust for roads too.