I’m a big fan of tricks that make my life easier in the kitchen and one that I use often is substituting a plastic bag for a pastry bag. A pastry bag is little more than a conical plastic bag that is open at the wide end and sealed at the bottom. To use one, you just need to snip off the closed tip after filling the bag and you can pipe batter into muffin cups or molds, or pipe frosting onto a cake. A variety of cake decorating tips can be dropped into the bag if you want your frosting work to be a little more artistic. Ziplock and ziplock-type bags have a lot of advantages and are very similar, design wise. Since the bags are square/rectangular, all the filling needs to be directed towards one corner, which can be snipped off just like the corner of a pastry bag.
Zip-top bags are much more common and more widely available, so while I do use them a lot, a pastry bag really is a better tool when it comes down to it. The reason is that a pastry bag is much thicker than a standard sandwich or freezer bag and has less “give” to it. A regular bag will stretch and bulge with heavy or extended use and it’s a pain in the you-know-what to have a bag actually break along a seam while you’re trying to pipe royal icing on to a cake. A pastry bag will not stretch or give, even with a thick frosting and an extended decorating time. In a pinch, you can even rinse them out and reuse them – especially if you buy the more expensive bags.
I keep both on hand and still reach for the sandwich bags at least half of the time, but it is worth having a box of piping bags (usually sold at craft/cake stores) for those times when a thinner, cheaper bag just won’t cut it.