If you are married: Tell her you love her. Tell her often. Don’t ever get bored or tired or let it become a cliche (or a quiche cos that would be weird) and so mean it every time. But say the words. Don’t assume she knows. Tell her again.
If you are dating: Don’t tell her you love her, til you really mean it. And that might mean different things to different people but love comes with commitment. And so until such time as you are committed to her (and that doesn’t necessarily mean a ring on her finger but it does mean you know she is at least potential for it) in some way or form don’t say the words. Don’t rush to say ‘i love you.’ It is too important a statement and reality to just be blurted out. If you say it when you mean it she will appreciate it more. The relationship will benefit more greatly.
Oh and if you are married, why not tell her right this moment – phone, sms, email, write it in sand with your feet on the beach (in sand on your bedroom floor will not go down as well but you might!), but don’t think your actions are ever just enough. You have to mouthe the words.