I assume you're talking about the type of HRM that rest just underneath your bra, in which case they are very easy to adjust without lifting your shirt because of their elastic strap. Once you get it fitted around you the only adjustment you'd have to make is *maybe* hiking it back up if it feel down though that has never been my experience.
Life goes by pretty fast. If you don't stop and look around once in a while, and do whatever you want all the time, you could miss it.
2010 Fuji Roubaix 1.0
2007 Fuji Absolute 2.0