Hi, ejasmudar! You can draw straight lines:

1) give a line start point and hold SHIFT key down and then point lines end point.
(this is MyPaint native method and unfortunately do not work very well with N900)

2) or you can use brush like what i used with red couch:
Tone's Liner / TK Tones Liner (very fast way to draw plain straight lines)

3) use steady hand and practise... practise... practise and practise.

It would be nice if there is some sort of constrain lock
so if you hold shift key and draw or even some sort of ruler.
(so we can use pressure to vary strokes at the same time) wink wink..

TIP: There is "Live Update The Last Canvas Stroke" check box in Brush Settings
(you can vary line thickness, color, opacity, etc. on the fly)