Can you extend your own life with the death note? How to Use: LVIII 


By manipulating the death of a human that has influence over another human’s life, that human’s original life span can sometimes be lengthened. 


which means you can use the note to extend your lifespan in the example i gave:

have some one get the shinigami eyes by giving them your death note and have them write down your life span then give back owner ship of the death note to you again (use the note to manipulate them into doing so) for example lets say you die 1st arpril 2009 at 1pm

next find some one who will out live you, you could use the person who you manipulated into finding your life span we'll call this person mr.X

write down your own name and have the cause stabbed by mr.X after calling him fat at 1st april 2009 at 1pm

next write down mr.X's name in the death note and the cause of death is a heart attack just before stabbing some one on the streert who called him fat on 1st april 2009 at 12:58pm

now if this would work you should get his remaining life span. Would this work?
No, because of several things:


A human with the Shinigami Eyes cannot see the lifespan of other Death Note owners. (How to Use: XVIII)
If you write your name and a cause that cannot be possible (being stabbed by a dead man), you will die of a heart attack. (How to Use: LV)
Writing your own name in the death note bypasses all lifespan calculations. If you wrote that you die at a certain date, you will die at a certain date, regardless of how much lifespan you have remaining.