Plan ahead and mark it down on your calendar just like you would if you're going on a date. This way, don't let excuses such as forgetting or having to go for other activities cause you to neglect it.
You can even set pick days in the month that are meaningful to you or your loved ones. Just like how Singapore now commemorates Racial Harmony Day on July 21 or the anniversary of the 1964 race riots, you could find similar anniversaries related to what matters to you.
Some good dates to go to the gym each month could be the date you signed for your membership, or you could also pen in the dates you met your significant other or had your child. This will not only help you remember anniversaries and birthdays, but also remind you why you're trying to be healthier.
The basic idea of making dates with yourself is to ensure you have a schedule and just need to stick to it, rather than to relying on your willpower each week to find time to go to the gym. You shouldn't be overly ambitious at the start, trying to go from zero to gym bunny in a week. Rather, it might be more encouraging to gradually tweak your plans as you go along and fulfil your commitments.