Lilacs do have some simple, but definite requirements to perform well. Onces these are met, they are extremely easy and durable shrubs. Without any guidelines about your specific conditions, lilac growth or the kind of lilac, I can only give you general advice. Sorry.
1 --- Lilacs must be in soil that does not stay wet. If you land is well drained, then you're good. But heavy clay soil (that does not drain quickly) can be fine, too, as long as the lilac is on a slight (or heavy) slope so water cannot pool when it rains and you don't artificially water on a regular basis. Lilacs like drier soils, much drier than most shrubs grown in the eastern half of the U.S. After the first season after planting and except for prolonged drought periods, lilacs should never require additional watering. Even in drought, never water every one, two, three or four days in clay soil. Water long and deeply once a week at the most.
A common mistake is planting a lilac deeper than it was originally, and this exacerbates water issues. When it doubt, always plant higher than intended. Remember that freshly dug soil settles.
2 --- Lilacs must have at least a half day of full, direct sun to bloom satisfactorily. The more sun, the more flowers.
3 --- If pruned, lilacs have very specific requirements to bloom well. Elaine's vague advice is correct if you understand what is meant, but her terminology is wrong. I'm going to give a lot of technical mumbo jumbo, so if you'd rather, just scroll down to the "take-away" advice for what to do regarding pruning.
Lilacs bloom on old wood. This means that this year's flowers (for 2015) were initiated as buds that were produced last season (2014). In other words, when your lilac grew in 2014, and produced the buds that are going through the winter right now, these are the buds that will produce flowers (and leaves) this year (2015). Similarly, buds that will be produced this summer (2015) will flower in 2016.
Most woody plants are not as picky as lilacs, when it comes to pruning and flowering. Lilac growth stops at the meristematic level within a few weeks or so after flowering. This is when new buds for the next season's growth begin, way down inside the twigs. This is when the lilac "decides" whether it is going to produce flowers (or not) for the next season (which is almost a year away!). So to not interrupt the flowering cycle, one can only prune within a two week time period after flowering. Pruning within this period will allow the lilac to produce new flowering buds that will flower in the following year. Pruning after this period will cut off these flowering buds that would have produced flowers the following season. Consequently, no (or very little) flowering will occur the next year, despite healthy growth.
Pruning at the wrong time of year will not hurt or damage a lilac, but as explained, has a huge impact on flowering. So, for instance, pruning in the fall will not kill a lilac, and pruning stems that would not have flowered anyway will not impact flowering.
TAKE-AWAY --- The only time to prune lilacs and still have flowers is the two week period after flowering has finished.
4 --- Lilacs do not need fertilizer, but it can help. If your lilac is in your lawn and subject to lawn fertilizing with a normal high nitrogen fertilizer, once or twice a year is okay. More than that can have a detrimental impact on flowering. If you fertilize a lilac intentionally, never use a fertilizer with more than double the amount of nitrogen (N) compared to phosporus (P) or potassium (K). It's better to go with a balance fertilizer, with approximately equal amounts of these three major nutrients. Remember to include the lawn fertilizer in your calculation, if that is a factor. If your lilac receives lawn fertilizer, it will never need additional feeding.
Bruce!
