I feel like choosing just one secret is too narrowing & there are ones that are closely related & interwoven. Take what you have is more important then what you lack. Now add do hard work needed to find a simple solution. When your limited to a certain amount of assets, you pretty much automatically have to work harder to find a way to utilize the alternative materials which is subsequently simplifying the process by just using what you have.
Delving in furthur, sticking to your immediate resources & simplifying makes it inexpensive. I needed two tables for my new apartment. The options were
Buying a table
Buying wood + materials
Scavenging + using what I had
In the end, keeping a eye out (peripheral vision?!) for "waste" like perfectly usable wood, nails & screws & obtaining a table from freecycle (for the base of one of them, these are rather vertical) enabled me to make two two-level tables for $5.00. I had plenty left over for other projects + more levels(coming soon)
Seeing the obvious (good, solid resources being disposed) & interrupting this unnecessary waste stream while at the same time benefiting from this personally is helping you & the environment. Sharing about this knowledge will make other people realize how much they could create themselves (much more fun in almost all cases) from recycled, typically free materials, rather then buying empty, unsustainable products. This is continuing innovative progress.
Try baking your own bread, installing a greywater and/or rainwater catchement, growing a garden, eating less meat & figuring out an alternative transportation method. In a personal effort to simplify, I am selling my car, instead riding a bike around. Try something that simplifies, making it less expensive at the same time. Then share your knowledge! I am always striving to work towards a minimal lifestyle. This is aided very much by shared knowledge. Evoke is a great example of how much of an impact shared knowledge can have. It's quite exciting.
Well, that's the way I see it. These secrets just seem to lead from one to another in a smooth flow.