Usually, try using a string that best matches the model and make of the modem, and make sure the correct modem is installed in Windows. Doing this will ensure the correct string. I know these are only the basics, but it's been a long time since I've messed with dial-up. Not to make you jealous, but I've been on DSL for a couple of years now. When I did use dial-up, my default string settings worked quite well. I've never really tweaked with the string commands.
Other factors of frequent drop outs would be poor line quality (audible noise), someone picking up the phone while it's in use, or sometimes the overall load of your ISP.
We're still trying to figure out why it takes 20 some tries to even get online on AOL at work, but another computer with the same software version at the same location works on the first try. I tweaked some settings on the problem one, thought it fixed it, but I don't think it did.
Also, if you're using an ISP such as Compuserve, AOL, or any other that requires special software to connect, it may fix the problem by reinstalling the software.
make sure you disable call waiting before connecting.. I remember getting disconnected because of an incoming call via call waiting.. use the *70, code in your dial up settings.. mind you, that is if you have call waiting..