Under Preferences, you can turn off the "stop on release" feature if desired, and you can set a Maximum value and Maximum change% for the throttle sliders. If the throttle slider is too small for your fingers, increase the size in Preferences. You can also set the initial web page for the full-size web panel, and enable the smaller web view on the throttle screen. There are also settings to help maximize the use of your device's smaller screen by hiding items not relevant to you.
Use the Function Buttons menu option to set the default labels and function numbers to display. If you have entered labels for a roster entry in JMRI, these labels will be used for that entry, and can be copied from the dialog (access the menu after selecting a loco). Remove buttons by blanking out the labels.
On the server side, check your preferences/settings to insure you allow or disallow the control features desired. If you've defined your turnouts and routes in a panel file, make sure to specify "user names" for those you'd like to see listed on your throttle. You can also "filter" the turnouts shown using the WiThrottle->Filter Controls option in the WiThrottle window.
Entering roster entries, and defining routes and turnouts in JMRI will all make your Engine Driver throttle more powerful and easier to use (though not required). EngineDriver will show additional info if the JMRI Web Server is started (such as roster details and icon images).
Note for MRC users: David Fuller has provided some additional setup information [here].