Amazing Photoshop selections from alpha channels, how to create them. The channels panel (found in the window menu) can be used to load and save and store data from the existing entries. If you open a RGB (or CMYK etc) and then go to the red (or cyan) and then go to the bottom of the panel and you will see four buttons (load, save, new, delete) and you can use them to create all kinds of selections and save data to the panel for future use (especially as they are saved with the document in PSB or PSD format).
The first one to click is the load button and if it is the red then the red pixels are suddenly the selection (nothing has been added to the panel at this point), if you are on the RGB then that is used to create the marching ants; or if you are on the blue then that is used.
You can then save it by clicking the next button but you can also edit the selection and then save it and it is saved to the panel as a new entry (not sure how many channels can be saved).
You can go to any of the entries and edit them as you would with the red or RGB. You can apply filter effects such as blurs etc. You can also use a small selection of adjustments such as levels to tweak the entry so you get more or less once you re-load it via the load button.
You can repeat this over and over to build up a library of selections that can be used via the load button (limited functionality) or by using the select menu and load selection which allows you to create a new selection or add to an existing or subtract from etc.
You can also click the new button and that just creates a blank white entry in the panel and you can then modify that with effects as well as gradients and brush strokes etc and then use that via the load command.
You can also right click any of the entries and delete them or duplicate the entry (if you duplicate the red then that is not the new red, it is just another entry that can be manipulated in the panel by load and save etc). You should note that all the newly created entries are alpha 1 etc but you can always double click it and change it to something more sensible.