Take many practice tests at home. While taking it, imitate test center conditions. For instance, take the practice test at the same time as your GMAT appointment time, take AWA's and try to do it till the last minute, make use of optional breaks and be precise in using them for less than 8 min, take good healthy snack and drink during breaks.

If you follow these, your stamina will definitely improve. good luck!

My debrief at the following link contains some strategies to build and maintain your focus while preparing for the test.

https://www.beatthegmat.com/my-debrief-f ... 54581.html

Hope that helps you.