From your question, and some of your responses to comments, it sounds like you are relatively proficient playing in all the keys, and in improvising, but you find yourself modulating back towards C over the course of the improvisation, regardless of where you start. Am I interpreting you correctly?
If this is indeed the case, then you may want to figure out how and why these modulations are occurring. Are you using pivot chords to shift from one key to the next subtly, or are you changing all at once, like the infamous "truck driver gear change" modulation? Are you doing it accidentally because the chords just seem to end up leading you there, or are you deliberately doing it because the piece sounds boring if you don't?
A few things you may try doing: while improvising melodies in a non-C key (perhaps Gb or Db!), limit yourself to only a handful of chords. Maybe start with just I, IV, and V. Maybe throw in the occasional ii or vi. But try to limit your improvisation around those chord structures. When you find yourself using a chord outside this set, stop, backup, and find a different way to proceed. If you really want to limit yourself, improvise the melodies only using notes from the pentatonic scale, which avoids the notes that are most actively used in several common modulations. Sometimes less is more.
Another possibility is to improvise around a simple existing melody, such as a hymn or a pop song, which stays in a single key. You can be more free with harmonizations, but if you stick close to the original melody, you'll eventually need to work your way towards a cadence in the original key.
If you find yourself modulating eventually because you get bored of playing in the same key after a while, try finding other ways to make it interesting by changing the texture. One option is to use registration changes -- play an octave higher, or play the melody in the left hand and chords in the right. Or switch between block chords and arpeggiations. By varying the texture rather than the key, it might keep the piece from feeling like it needs to modulate to stay alive.
If you absolutely must modulate, and don't want to go towards C, then try finding keys that are further away from C, and deliberately modulate to them.
To a certain extent, I think it's only natural to think in terms of C on a keyboard, since it is the most natural (ahem) scale to play. Sometimes, I still find myself thinking of music theory and chord relationships in terms of their C-equivalents, and needing to "translate" them. The best thing for that is to think of chords and scales in terms of numbers rather than letters.
Also, if you aren't already familiar with the concept of functional harmony, you may want to look into that, since its a way of structuring chord progressions in terms of their tonic goal, by recognizing each chord has a one of three functions (pre-dominant chords, dominant-like chords, and tonic-like chords). It's sort of a grammar for chord progressions.
This is a bit of a shotgun answer, since I'm not sure of your exact situation, but hopefully something here hits.