I think you need to exit out of the game and re-start it for changed settings to take effect. For example, if you start by loading Frogger with default dips, then change lives to 5, you're still only going to get 3 lives until you exit out of the game and restart it.
I really enjoy Frogger, but oddly I have not played very much of it at all since building my MAME cabinet 6 years ago. I kind of forgot how fun this game can be.
Frogger is almost like 2 games in 1, because navigating the bottom half of the screen requires a whole different set of techniques than navigating the top half.
For the first 2-3 boards, IMO you don't even really need any kind of strategy--the only thing you need is patience and restraint. Take your time, make reasoned and low-risk jumps, and you should be fine. Most of your hopping on these boards will be forward; not much side-to-side movement is needed.
When the traffic on the road starts getting more hectic, two things start to become really important: the first is learning how to hop side-to-side between two cars, "in sync" with the speed of the cars of that row so that you don't accidentally hit the one in front of you or get squashed by the one behind you. Hopping along within a row of traffic in this way allows you more flexibility in choosing the exact moment you want to make the leap to the next row up, rather than always waiting for an obvious vertical track from bottom to top to present itself.
The second important thing on hectic roads is learning how to look ahead--not just to the next row above you, but beyond. The reason this matters is because, if you look closely, you'll notice lots of times that rows of cars have gaps in them. These gaps present great opportunities to hop through at lower risk. If you take mental note of where those larger gaps are, you can try to time your advance so that the gaps pass in front of you just as you need them. I could never get past about 16,000 points until I figured this out.
I agree with MO's suggestion to fill the left-most slot early on, but I don't think you need to necessarily do it first, just do it at the earliest opportunity. As soon as you get past the road, check to see if you think you have enough time to plot a path to that slot--if not, take an easier slot and try again next time for the left side. You don't want to lose a life from running down the clock if it can be avoided.
Most of the time, death by sinking turtle is completely preventable. A quick scan of the turtle rows in the water should be enough for you to identify where the sinkers are. Try to make a conscious effort to always identify the sinkers before hopping into a turtle row.
Frogger was designed to work with a simple 4-way joystick, which would make one think it would be easy to emulate at home, but I find that most controllers don't work very well with this game. The reason is, you move your frog with a string of discrete taps rather than the usual method of just holding the stick in the direction you want to move. The real arcade machine had a red ball-top stick with a short throw that was pretty springy, which made it very suitable for rapid tapping in different directions with accuracy. Lots of generic arcade joysticks and hand-held controllers are not very good at mimicking this kind of performance. That's why Frogger might be the only game in the MAME library that I think plays well with a keyboard; the tap-tap-tap inputs translate quite logically to key presses. The thing to keep in mind when using a keyboard is that the game will only register inputs as quickly as the frog can jump, so you have to get into a rhythm and avoid the tendency to rapidly mash the buttons to jump quicker. If you do that, you'll get lots of unregistered button presses and you'll be likely to either over- or under-jump to your death.