Balancing your freelance with your day job

I like my job, I like having a paycheck every 2 weeks and I like all the benefits that comes along with that job. I also like to freelance it fills a void that my job may not offer and puts a little extra cash in my pocket but it's not a stable income. Knowing this I am not real aggressive about looking for freelance gigs. I have been lucky to have some nice clients fall in my lap. I'm sure I could do a lot more if I got out there and marketed myself. I do let my freelance clients know I do have a day job so that comes first but I'll do everything I can to work with their needs.

I'm a big believer of not letting freelance work get in the way of your day job, but sometimes a good opportunity shows up short notice and it's hard to say no. When that happens what do you do? Do you beg for the day off? Do you call in sick? Do you make up something to get out of work?

This is what I did
I got a call from the client (time.com) they wanted me to shoot a few interviews and some supporting video for a story they were working on. It was a Thursday and they wanted to shoot on a Tuesday.   Holy cow! I need to get a day off and now! What do I do? Do I say nothing and call in sick or do I just ask for a day off with short notice.  I walked into my boss's office and told him I was sorry  for the short notice told him what I was doing and how good of an opportunity shooting for Time was he agreed and granted my time off.
I know that I don't have to tell my boss why I need the day off but being straightforward worked for me and I do believe that because I was honest with my boss it allowed me a little more freedom when it came to asking for time off.
If I would have called in sick and did the shoot I could not have told anyone I work with about how cool it was to be asked to shoot for Time.com.

It's always better to be truthful and keep your trust with your boss and your clients.