Needs["Graphics`PlotField`"];
l=100;
modell={u'[s]== - u[s],
v'[s]== - 2 v[s],u[0]==u0, v[0]==v0};
p:=Module[{},loesung=NDSolve[modell,{u,v},{s,0,l}]; 
abb=ParametricPlot[Evaluate[{u[s],v[s]}/.loesung],{s,0,l},
DisplayFunction->Identity,
PlotPoints->50,PlotRange->All]
];
bild[1]=Table[{u0=z;v0=-1;p},{z,-1,1,0.25}];
bild[2]=Table[{u0=z;v0=1;p},{z,-1,1,0.25}];
bild[3]=Table[{u0=-1;v0=z;p},{z,-1,1,0.25}];
bild[4]=Table[{u0=1;v0=z;p},{z,-1,1,0.25}];
feld=PlotVectorField[{-z1,-2 z2},{z1,-1,1,0.25},{z2,-1,1,0.25},
DisplayFunction->Identity];
abb0=Show[Evaluate[bild[1],bild[2],bild[3],bild[4]],feld,
DisplayFunction->$DisplayFunction,
PlotRange->All,AxesLabel->{"x","y"},ImageSize->{400,300}];