You cannot hold your best speed for a 100 over and over again in practice - if you did, then you should be capable of going faster for a single effort 100. You can hold the same SPEED as you would for a 100 over a shorter distance, like a 75.
My favorite set to build good speed for 100's is two rounds of 6 x 75 with some easy swimming between each round. The 75's are done at the best speed that can be repeated for all 6, with rest equal to approximately the same time as the swim.
For example, based on experience, I know that within the last few weeks the best time I can do for a set of 75's is a 1:07, then that is my goal time for each swim. I will swim the 75, then rest for about 1:07 - the 75 should have taken me 1:07 to swim, so I will take an equal amount of rest (rounding by a few seconds is OK).
The set might be done as 6 x 75 on a 2:20 interval, aiming to hold a 1:07 on each 75. After swimming 6 of the 75's, then swim an easy 100 focusing on technique, then take another few minutes of rest. Finish with a second round of 6 x 75 on 2:20, aiming to hold a 1:07 on each swim.
This set would be done as part of your regular training day or week, probably once each week for 3-6 weeks. Make sure you are fully warmed up and that you cool-down afterward. Swim On!