Try using 1 ml Gilson pippete tip at the site of tail stab and let the blood flow using gravity. Put on ice straight away and spin as usual.  I have used this technique to harvest serum for Renin assay. worked for me.

Be sure to make sure the animals are warm (eg working near an incandescant lamp).

Thanks Khalib ansd Alexander. How fast and how long to you usually spin the samples?

Hi Aymeric, I would have thought 3 mins would have been sufficient at the rpm you are employing (maybe even too high). Have you optimised this protocol from the assay instructions? Could the hemolysis be a result of an osmolarity inbalance somewhere along the way rather than in collection technique. Try collecting blood using scapel cut in the tail vein and using EDTA tubes maybe?? Hope you find an answer!

You can collect the blood in drops from tip of tail using a very small cut. further for serum/plasma collection, pl see  Harris et al., 2007 Infect. Immun. 75(5): 2366-73 

I agree with Alexander. I recommend you maximally 2500 to 3000 xG spinning for plasma or 1500 xG spinning for serum, for 10 mins. I do not know how it is in rpm on your centrifuge, it always depends on a rotor radius. Probably too high acceleration destroys erytrocytes in your samples.

Another thing is that in your tubes the SST clot gel activator is used. It means that you should allow sample to clot after gathering for at least 10 min and do not shake, invert only after sampling, to avoid hemolysis. 

For computing Relative Centrifugal Forces (G) from rpm and a rotor radius volumes please see this norogram. I hope that it helps.