Proper wick trimming and a moderately sized flame should result in a vigil lamp that burns nicely for many hours before needing to be re-trimmed.
1. When putting in a new wick, make sure that it is the correct thickness for the wick holder you are using. If it is too thin, it will fall out of the wick holder. If it is too thick, the wick will expand from saturation with oil and won't be able to pull the oil up through the wick holder, resulting in a poor flame. The wicking sold by the Hermitage is the ideal size for the Old Rite wick holders. However, one or two strands may need to be removed for those using the brass wick holders. Also, before inserting the wick into the wick holder, soak the tip of the wick in oil so you can thread it through the wick holder easily, and so that it will burn well when you first light it. If it's dry, it won't produce a good flame.
2. We suggest obtaining a pair of small sewing scissors for trimming purposes. Tweezers and tissues can also be used, but we have found that small sewing scissors work best. Every time (or almost every time) you light the lamp, trim the black carbon tip off of the top of the wicking. Make a clean horizontal cut at the bottom of the black tip. It should cut easily. If it feels resistant, chances are you are trying to cut through the unburnt part of the wicking, which isn't necessary. Make sure the cut is flat and clean; if the cut is rough and uneven, or if you pull away the carbon tip with a tissue leaving strands of thread, it will very quickly result in a carbon tip (looking like a little wishbone in the midst of the flame) that dulls the light of the flame. These carbon tips can also tend to fall into the oil of the lamp, and when these build up they cause the lamp will look dirty and make it messy to clean.
3. If you do get carbon tips, you can easily remove them by running the two scissor blades up the wicking and lifting off the carbon tip while the flame keeps burning. You'll notice the flame will look and burn better after that, but you'll still need to re-trim the wick sooner than if you had trimmed it properly.
4. When you trim the wick, a good length to keep in mind is no. 2 pencil eraser heads. That's the length, and that's the flat top that you want on your wick so that you don't get carbon tips. If you do that, you can light your lamp in the evening, go to bed, and when you get up in the morning, your lamp should still be burning nice and clean.
5. If you are successful at lighting your lamp, but after a while the wick just doesn't seem to be burning as well, it can be one of two things. Either change the wick out for a new one (making sure that it is of the proper thickness), or try using different oil. Most of the time it's the wick, but once in a while it will be bad oil. Olive oil is the traditional oil to use in a vigil lamp. Vegetable oil will also work, though it is stickier and messier than olive oil, which is clean. Avoid extra virgin olive oil, however, as this does not burn well.
6. When trimming the wick, have a set of tissues nearby; you can use these to clean off the carbon tips and oil from the scissors. Keep a little bag or can for the used tissues, and burn the trash when it becomes full in a place where you burn sacred things. You may also be able to take it to your church to burn; most Orthodox churches have a place where they burn 'holy trash'. Vigil lamp oil when burned in front of your icons becomes sanctified (it is often used for anointing oneself or others), so things used to light and clean them shouldn't be thrown out with the common garbage.