I am tired of lousy launch ramps.
Bad launch ramps aren't usually that way on purpose. They're usually lousy because, when they were built, nobody involved knew any better. In the interests of making life better for all trailer boaters, then, here are some pointers on how to build one correctly- and a free plan you can give to your contractor as a starting point for your own ramp design.
(This drawing is free-as-in-beer, free-as-in-speech for all purposes under Creative Commons CC0. Matthew Marsh and M.B. Marsh Design assume no liability of any kind for any losses you or any other party may incur in connection with the use of this drawing.)
4.5 m (15') per lane is ideal; a bit wider certainly won't hurt if you can afford it. Don't go much narrower than that- many trailer boats are 2.6 m (8'6") wide, and many people find it difficult to handle a trailer in reverse with less than a metre of clearance on each side.
A slope of 12% to 15% (7° to 8.5°) is ideal. On a ramp shallower than 12%, a typical car will have its rear axle in the water by the time the boat is in far enough to be pushed free. Any steeper than 15% and people are likely to slip and fall, plus it's hard for front-drive vehicles to get enough traction on steep, wet slopes.
Concrete ramps should have drainage grooves trowelled into their surface, sloping away from the centre of the ramp.
The foot of the ramp should be about 1.2 m (4') below the surface at low water. Many places won't give you a permit to go any deeper (and there's rarely any need to), but if the foot of your ramp is too shallow, it'll need constant repairs as the surrounding gravel is eroded by prop wash.
Height Above High Water
Don't level off the ramp until at least 0.5 m (1'6") above high water. It's OK to go higher, or to level off gradually, as long as the staging / turn-around / parking area is high enough that waves won't break into it.
The paved part of the ramp should always extend at least 2 m (6') onto the flat region, even if everything else is unpaved. Front-drive cars tend to spin their wheels if they hit gravel before they've passed the crest of the ramp, which is a nuisance to drivers and a maintenance headache for the ramp owner.
Side docks are not strictly necessary on very small ramps, but they do make life a lot easier. (If you plan to charge a fee for use of the ramp, it had darn well better have a good side dock!) A good side dock should float (or be adjusted) up and down with the water level. It needs to be secured well enough (or be wide enough) to not be tippy. Anything narrower than 1.2 m (4') is asking for trouble, as these will be busy docks with a lot of people on them- very much unlike the narrow finger piers of a marina.
The side dock should have hefty cleats, 200 mm (8") or bigger, ideally placed every 2.4 m (8') although, if you're cheap, you can space them farther apart. Single-handers, or those without competent crew, need lots of tie-off options to keep the boat in exactly the right place while they back the trailer down the ramp.
Your contractor can help you select a concrete mix, slab thickness, sub-grade preparation (gravel, etc.) and rebar schedule that's suitable for your location. It makes no sense for me to try to recommend something specific- something appropriate for a busy marina in Texas would be far too costly, and not nearly ice-resistant enough, for a quiet Northern Ontario lake.
Look for a concrete supplier who has experience with shoreline work, even if they are more costly than the alternative. Concrete mixes that are appropriate for shoreline work are very different from what you'd use for a house foundation or a patio.
Almost any location in North America, Europe or other developed areas will require some form of government permit- often several permits from several levels of government- for you to put in a boat ramp. Don't skip this! If in doubt, the closest city hall is usually a good place to start.
While there are undoubtedly a few bureaucracies that are just out to slow you down, most places run their permit review process with the intent of making the project better. They'll often catch things that you would never have thought of. For example, your proposed site grading scheme might cause your lake's best fish spawning grounds to silt up. Or perhaps your ramp will disrupt the river's current in a way that would cause the ramp itself to be constantly buried in muck. Talk to your local officials well in advance. Heed their advice. Make sure your plan addresses their concerns.