Maneuver Node Improvement plugin exists to assist in carefully designing maneuver nodes and dealing with their slippery nature. With the the plugin you can easily cycle through the available nodes and edit their attributes either with the help of keyboard or mouse.
Originally released somewhere around 0.17 I guess, I have not updated this plugin for a while. The new version adds buttons to enable mouse-only usage, buttons for changing patched conics mode, and a toggle switch that will allow you to edit the previously active node even if the bloody Gizmo closes. Try not to double-click too much, or you're going to change the camera focus to something else.
The key you use to cycle through maneuver nodes is by default O (as in Orange). Pressing this key opens the last node you had open before it closed. If no node was open before, it opens the first node in the node list. If a node was alreadyd open, it opens the next node in the list. So if you accidentally close the node you were editing, just hit O to bring it back open.
Using the window, you can adjust the universal time when the node happens. Pgd stands for prograde (green buttons on the node gizmo), Rad stands for Radial (blue buttons on the gizmo) and Nml stands for Normal (purple buttons on the gizmo). You can edit the node values with your keyboard or by using mouse. If you use mouse, you should tick the "Keep open" toggle to ensure that the window doesn't close when your node closes (KSP feature, no can do!). There are four buttons: -, a number, + and S. Use - and + to adjust the number. Use S to flip the sign of the number (positive/negative). Click the number to add it to the node.
The window also displays total Delta V required for the node and you can quickly change patched conics mode. Changing patched conics mode is very useful when you're trying to create an accurate encounter. You can set camera focus on the destination planet and change the conics mode to help you visualize the encounter.