One of my ice candles broke as I tried to remove the plastic cups - they are very fragile. See original method here.
The answer is to use two different sizes of cup. (so obvious really!) This has several advantages. It is easier to fit leaves etc between the two layers, it makes a chunkier candle which you can briefly dip in hot water to release from the cup, and it lasts longer. Not quite as delicate perhaps but still rather pretty.
You can remove them from the cups once frozen and put them back in the freezer for instant use at any time.