When you look around the internet, it is very clear that there are many people out there that have great ideas.  The place where a lot of these ideas stumble is in the execution of actual project.  The build of a particular device often includes a lot of pitfalls, or stumbling blocks, that could possibly have been avoided with a little more careful consideration and planning.  That said, there are a million things you could watch out for, and would ultimately keep your project from ever getting off the ground if you tried to adhere to them all.  Lets talk about a few important considerations for your project's design, that will help it be successful and easily reproducible.

1. Clearly Define Your Requirements:
   Specify exactly what your goals for a product or component are, including things that must not be done, in addition to those that must be met.  Don't go overboard, because there are an infinite number of constraints you can place on yourself, but having a list of concise criteria will help you a lot.  Consider carefully who will use your finished project, and in what way.  Will someone need to be able to pick it up and carry it, which would limit size and weight?  Would it be ok if it had wheels instead of needing to carry it completely off the ground?  Those are just a couple samples of ideas that might guide your specifications.
2. Don't Reinvent the Wheel:
   It is often very easy to get caught up in designing, and fail to realize that you just spent a lot of effort to design a component that is very similar to something that already exists.  There are certainly times where you will need to resign a common part in order to achieve a specific necessary goal, but do your best to keep these instances to a minimum, and recognize when they need to occur.  For example, do not assume you need to solder your own battery pack, since there are thousands of companies that specialize in creating cost-effective battery packs for R/C cars, cell phones, etc.
3. Look to Other Industries for Components and Inspiration:
   Other people have already encountered your problem, and most likely solved it, but for completely different reasons.  It's humbling to accept, but often you can draw inspiration or parts sourcing from unrelated fields that have run into the same design dilemma that you are facing.  To continue with our battery pack example, R/C car companies have been fighting with the issue of making powerful battery packs affordable to younger consumers on a budget.  So if high power, inexpensive batteries are what you need, try to think of an unrelated product that faces the same issue of drawing lots of power but needing to be sold inexpensively.
4. Sometimes It's Cheaper to Buy a More Complex Part and Disassemble It:
   This one always boggles my mind, but you can often find a component you need bundled into another product cheaper or easier than you can find it alone.  For example, it may be cheaper to buy an entire battery charger, that has the connector to attach your battery, and just cut off the connector, than it is to buy the connector alone.  This can be caused by lots of things - buying in bulk, supply and demand, sourcing products from overseas, etc. - but if you think the price for an individual component is high, don't be afraid to look for it packaged into another product.
5. Try to Design Out the Need for Special or Expensive Tools:
   If you expect people to build your project, because you have given instructions or sold a kit, make sure that it requires as few one-off tools as possible.  Spend extra effort to come up with alternative designs that minimize the need for such things.  Sometimes the need for a particular tool is unavoidable, or it's even possible that you are creating a design specifically for people that have a special tool (e.g. CNC machine), but try to minimize extraneous uses of odd tools, because they add needlessly to the total cost of the project.  For example, avoid the use of holes that require an odd size drill bit to replicate, especially if the bit is not commonly found in typical value-kits (e.g. 1 1/8" forstner bit).
6. Try to Create a Balanced Design, Free of Disproportionate Bottlenecks:
   Identify the bottlenecks of your design, and do your best to adjust them so that no one component or subsystem is holding back the rest of your project significantly.  Conversely, do not spend effort correcting a "problem" that can not actually occur in your design because of other limitations.  An example would be putting racing tires on an economy car.  The rationale against doing that would be that obviously the tires are not the limiting factor of the car's performance, which is probably due to a small underpowered engine.  You would ideally want tires on the car that were just good enough handle the car's typical intended uses (e.g. Z-rated, 149MPH+ rated tires, don't provide much good to a car that cannot top 100MPH).  As you become more aware of this, you'll notice that much of the aftermarket accessories industry focuses heavily on getting you to pay lots of money for theoretical improvements, that you often will only receive trivial benefits from, if at all.
7. Treat Root Causes, Not Symptoms:
   It is easy to look at a problem only on its surface, and create a fix for the issue you are directly experiencing (i.e. "symptoms"), while overlooking deeper circumstances (i.e. "root causes") that lead to the situation causing the symptoms.  This often happens because the symptoms are easier to address than the root problem, but it also often just leads to an increased number of issues later on.  This point seems like common sense in some circumstances, such as if you had a pot of boiling water, you would turn off the heat instead of trying to increase the pressure (increasing the pressure far enough would keep it from boiling).  In other situations people turn a blind eye to the core issue, such as putting a larger engine in an unnecessarily heavy vehicle, instead of trying to shed weight in ways that do not reduce functionality.
8. Source Your Components from as Few Sources as Possible:
   It's easy to think you're getting a great deal by buying each component from the cheapest place you can find it, but often the extra shipping fees add up to far more than you would have paid by paying even inflated prices at a single vendor.  Many companies specialize in products for a particular field, and even if their prices are slightly higher than the "best deal around", it could cheaper to get them in one place.  Having fewer sources also saves your sanity, because managing many different parts suppliers can easily lead to added complexity and human error.

These are just some of many points you could use to guide your designs, but hopefully I've been able to hit on a few of the important, yet often overlooked ones.  Keep turning out your awesome projects, and feel free to share any other ideas you have in the comments below!