I use three different adhesives for the most part. My most used one is Tombow Multi - SU sells it, as do many other places. It is the most versatile and most cost effective - it seems to last forever and ever and ever. I use the flat side the most, and then there is a little skinny tip side that is great for small letters and such.
Next I use mini glue dots a fair bit. I would use these for sticking on buttons or other small embellishments and sometimes if there's a lot going on on the page and I don't want to deal with wet glue, I'll use them for my picture layers too. They're not inexpensive, but they aren't super pricey either. They last a long time and once they're stuck they are stuck!
Finally, for ribbon on scrapbook pages I will sometimes use Sticky Strip - it will stick when glue dots won't.
Hope this helps!
Paper Crafty Goals for 2013: 50/135 pages and 185/75 cards.