Same way you stay at any job after the point when you're bored with it:
1) convince yourself you like it ("find the fun in it" is what I tell my kids about school stuff they complain about.)
2) Set up a little spreadsheet that tells you how much an hour or a day or a month of extra work is going to give you in terms of extra annual income for the rest of your ER life. (Salaried worker's form of watching the clock)
3) Pick fights with co-workers to keep your adrenaline up and your attention engaged (sorry, being facetious on this one
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4) Ignore the whole thing for awhile and just keep slogging away until another year is past.
5) Disengage mentally in stages by pursuing other stuff that is along the lines of what you'll be doing in ER, so that work is done with less and less of your real attention.
6) Take longer lunches, leave earlier, goof off more, take advantage of more of the discretionary travel, training, conferences etc. (Post here from the office
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7) Look for a sinecure -- a new job within the organization that is interesting but not too challenging.
8 ) Reinvigorate your career with a promotion and new challenges -- as long as you're there you might as well do it really well and gain the maximum amount of experience, income, excitement.
9) Try to figure out something really good for the organization that no one else has the time or talent to figure out -- a way of saying thanks, or ending on a high note and giving them something useful before you go.
But the way I personally did it was to just make myself go numb every day on the way to work -- 'suck it up', I told myself. I actually used to have these involuntary visions of injecting myself with some sort of 'numb syrum' to just go in there, do the job for another day, and go home. It sucked, and it was very bad for my health, but it got me through the last year or two prior to ER. I was kind of a physical basket case by the end, though! If you have a way to leave earlier than that, I'd say it's worth considering.