"correct" way to include a custom template?

[DetroitYES]

I've been working on a particular skin for our site Detroityes.com that will include sidebar navigation to the rest of the site (the non forum sections) and google ads... You can see the testing server here:

http://www.atdetroit.com/testvb/forumdisplay.php?f=2

I was able to constuct this fairly easily by following the "How To Include a Custom Template via Plugins" Thread:

https://vborg.vbsupport.ru/showthread.php?t=119933

My question relates to a comment someone made about this not being the most efficient method of including a custom template. Which is the most efficient way?

My understanding of php is rudimentary but my instinct tells me that the template names need to be added to the array elsewhere, instead of running the function every time a page is loaded? In other words, how or where would I alter the php to as to tell the software to load these templates as it would the non custom ones? Would this be in cache_templates and global_start? Seems logical enough but I'm a little timid to fart with the php until I atleast know that I'm in the right spot...

Thanks in advance,