Suggestions

The output work is more than the input work, so remember to store energy at various points in the machine.
Consider building a frame for your machine, it will hold the components in place and make it easier to move (be sure to consult the rules for frame size limitations).
If the machine has any heavy falling parts, be careful not to let them fall on the base or anywhere else that they may set off subsequent steps.
If you have very complicated or hair-trigger steps, add safeties to keep the machine from being set off accidentally.
Remember, the machine before yours may pull the string harder or farther than it needs to; be sure your first step includes "break-away" parts which can be pulled out of place without disrupting the rest of the machine's operation.
If you are going to paint your machine, particularly if you are going to use spray paint, you should paint the individual parts first, and then build your machine.

Most paints shrink as they dry, so plywood, cardboard, or other flat, thin surfaces tend to warp when they are painted (especially if the paint is only applied to one side).
While it may feel dry to the touch after just a few hours, most paints take several days to cure fully. If the steps of your machine require fine-tuning, they may need to be re-adjusted several times before the paint becomes stable.
Spray paint (or any paint which has been applied carelessly) can cause parts to stick together. If you wait until your machine is otherwise finished and then spray it with paint, it will almost certainly get gummed-up and stop working!

Add time-delayed steps; they give your audience a moment to catch up with the action and add drama to the execution.
Change the direction of motion in the machine's steps, a one-dimensional machine is less entertaining.
Add parallel lines of action, but be careful about timing sequential steps.
Though it is not required, some groups choose to give their machine a theme. This can make a machine more entertaining for your audience, and it may even help you with when coming up with ideas for the steps.

A triangular block of wood is only a wedge if its triangular shape is used to separate two objects when the block is forced between them - if the triangle simply hits something and knocks it over, it's not acting as a wedge.
A central axis with threads around it is only a screw if its rotation causes a nut to move along its length - if a force on the end of a wood screw causes it to tip on a fulcrum, it's acting as a lever.

For A Simple Machine to Count Towards the Four-Machine Requirement, It Must Advance the Action of the Overall Machine By Using Its Mechanical Advantage!

If a board on a fulcrum tips up so that an object can roll up its length, that may be a cool idea, but it's an inclined plane, not a lever.
If a toy car rolls along on a set of wheels connected by axles, but no force is applied to the wheel to make a force exerted by the axle advance the machine, then it doesn't count as a wheel and axle.

if you use dominoes, keep the number reasonable (no matter how many dominoes you use, they are only part of one step)
be sure no steps have loose parts that can set them off prematurely
don't let heavy objects fall near them
try finding dominoes with squared corners; rounded dominoes fall over too easily

can store a lot of energy and can be set of easily, which makes them a good choice to re-start a machine that's winding down
can be used to produce a wide variety of outcomes

do not mount permanently (assume traps will need to be replaced several times as they wear out)
consider attaching booms to the trap's hammer to extend the range of motion
protect the trigger from lose parts set in motion by other steps
don't let heavy objects fall near trap

relatively weak/low durability
when removed, leaves a sticky residue which can interfere with moving parts (this is especially true of duct tape)
can be unsightly

use masking tape during testing phases, but consider going to more permanent solution as the machine is finalized
be sure to clean up and left-over adhesive fully when tape is moved/removed
if your final project includes tape, use it sparingly and apply it neatly
Never count on tape to fail! steps that require a piece of tape to release an object at a specific moment are always a problem (the tape either sticks too well or too poorly)